Semiconductor devices

ABSTRACT

An insulating member ( 4 ) of an amorphous structure partially opened to expose a substrate ( 1; 1, 2, 3 ) is formed on the substrate. At least a compound semiconductor ( 5, 51, 52 ) containing at least nitrogen as a constituent element is deposited on the insulating member ( 4 ) and the substrate ( 40 ) exposed by the opening thereby to form a semiconductor material ( 1, 5, 51, 52 ). A semiconductor material ( 6, 7 ) configured of the first semiconductor material or configured of the first semiconductor material and another semiconductor material grown on the first semiconductor material is processed thereby to form a semiconductor device.

This application is a Divisional application of application Ser. No.09/043,384, filed Mar. 18, 1998, now U.S. Pat. No. 6,377,596 which is anational stage application, filed under 35 USC §371, of International(PCT) application Ser. No. PCT/JP/02663, filed Sep. 17, 1996.

TECHNICAL FIELD

The present invention relates to the crystal growth of a III-V compoundsemiconductor composed of at least one of what is called the group-IIIelements including B, Al, Ga, In and Tl and at least one of what iscalled the group-V elements including N, P, As, Sb and Bi, or more inparticular to crystal growth techniques desirable for forming thecrystal of a structure having the hexagonal symmetry structure or thecrystal of a III-V compound (hereinafter referred to as “the nitridesemiconductor”) required to contain N (nitrogen) as a group-V element.

Also, the present invention relates to a semiconductor device composedof the crystal having a structure of the hexagonal system or anitride-semiconductor, and to semiconductor light-emitting diodes andsemiconductor laser devices suitable for emitting the light withwavelengths up to the ultra=violet ray or suitable as a light source foroptical information processings or a light source for opticalmeasurement equipments.

BACKGROUND ART

In recent years, various reports on the diodes and laser devices foremitting a light in the wavelength of blue region using GaInNIGaN/AlGamaterials have been published in Appl. Phys. Lett., Vol. 64, March 1944,pp. 1687-1689 (Article 1); Appl. Phys. Lett., Vol. 67, September 1995,pp. 1868-1870 (Article 2); Jpn. J. Appl. Phys., Vol. 34-7A, July 1995,pp. L797-L799 (Article 3); Jpn. J. Appl. Phys., Vol. 34-10B, October1995, pp. L1332-L1335 (Article 4); and Jpn. J. Appl. Phys., Vol. 34-11B,November 1995, pp. L1517-L1519 (Article 5).

What is shared by the semiconductor devices disclosed in Articles 1 to 5is that a buffer layer composed of the above-mentioned nitridesemiconductor is formed on a sapphire (Al₂O₃) substrate, and a nitridesemiconductor layer is grown on the buffer layer. Such a structure isdisclosed in JP-A-4-297023 (and JP-A-7-312350 constituting a divisionalapplication thereof, and a corresponding U.S. application patent No.5,290,393) and JP-A-4-321280. According to the teaching ofJP-A-4-297023, a polycrystalline layer is produced by forming a firstnitride semiconductor layer made of Ga_(x)Al_(1-x)N (0≦x≦1) on asapphire layer at 300 to 700° C. lower than the melting points of thesematerials. When a second nitride semiconductor layer is grown on thispolycrystalline layer at a temperature of 1000 to 1050° C., the secondnitride semiconductor layer is epitaxially grown with the grains(crystal grains) constituting the first nitride semiconductor layer asnuclei. As a result, an epitaxial film of a nitride semiconductor havingfine surface morphology can be formed on the sapphire substrate. Aproposal thus has been made to utilize the above-mentionedpolycrystalline layer as a buffer layer for forming a semiconductordevice.

The reason why a semiconductor device composed of the above-mentionednitride semiconductor is formed on a sapphire substrate is, as disclosedin JP-A-6-101587, that the crystal structure of sapphire, unlike that ofGaAs or the like (having a cubic symmetry structure of zinc-blendetype), has a hexagonal closed-packing structure (also called hexagonalzinc sulfide or wurtzite structure). According to this publication,however, the difference in lattice constant between GaN and sapphire isas large as about 14%. The nitride semiconductor layer formed on thesapphire substrate, therefore, develops lattice defects such asdislocations, so that the non-saturated bonding caused in the nitridesemiconductor layer forms a doner level or absorbs elements ofimpurities constituting donors. The resulting problem is that thisnitride semiconductor layer assumes N type and the life time of thecarriers injected into this nitride semiconductor layer is shortened.This publication, in order to solve this problem, discloses a techniquewhich employs a substrate made of MgAl₂O₄ having a cubic symmetry spinelcrystal structure or MgO having a NaCl-type crystal structure andfabricates nitride semiconductor layers on the substrate by matching thelattice constants between them. A semiconductor laser using thistechnique is reported in Appl. Phys. Lett., Vol. 68, April 1996, pp.2105-2107 (Article 6).

The above-mentioned conventional techniques teach the possibility ofrealizing a semiconductor device made of what is called a nitridesemiconductor required to contain N (nitrogen) as a III-V chemicalcompound semiconductor or a group-V element having a crystal structureof the hexagonal symmetry structure. Nevertheless, sufficient data (forexample, the continuously operating time of a laser device) are notavailable to guarantee the practicability of such a semiconductordevice. Especially, the supplementary trials conducted by the inventorsshow that the density of defects developed in the nitride semiconductorlayer is as high as 10¹¹ cm⁻², and the inventors judged that a laserdevice capable of being operated continuously for at least 1000 hourscannot be realized under the above-mentioned conditions.

In recent years, NIKKEI ELECTRONICS, Dec. 4, 1995 issue, No. 650, pp. 7(Article 7) has reported that as a result of a joint research madebetween Cree Research, Inc. and North America Phillips, it was foundthat the use of SiC crystal as a substrate reduces the lattice defectdensity of the nitride semiconductor layer formed on the substrate to aslow as 10⁸ cm⁻² and thus can realize blue laser diodes higher inbrightness than the conventional devices. According to Article 7,however, the defect density of the nitride semiconductor layer isinsufficient to lengthen the life time of the laser diode, and thereduction in the defect density (10⁴ cm⁻² at present) of the crystal ofthe SiC substrate is indispensable for reducing the defect density ofthe nitride semiconductor layer. In constructing a semiconductor laserby forming a nitride semiconductor layer on a SiC substrate, therefore,an improved quality of the SiC substrate as well as the growth of anitride semiconductor layer is indispensable for lengthening the lifetime, and the development cost is expected to increase.

Also, the articles and publications introduced above refer to theconfiguration of a nitride material used for an optical active layer oran optical waveguide layer but not to the shape of the active layer orthe waveguide for controlling the transverse mode of the semiconductorlaser. Thus, none of the articles and publications contain thedescription of a method of reducing the crystal defect density suitablefor the waveguide structure mentioned above or, especially, a method ofreducing the optical loss in the neighborhood of the active layer of thewaveguide.

DISCLOSURE OF INVENTION

A first object of the present invention is to realize a crystal growthtechnique for forming a semiconductor layer having a very low defectdensity made of a III-V compound semiconductor having a crystalstructure of the hexagonal symmetry structure or made of what is calleda nitride semiconductor required to contain N (nitrogen) as a group-Velement. “A very low defect density” indicates a defect density on theorder of 10⁷-cm⁻² or less which is difficult to attain by theabove-mentioned technique using a SiC substrate. This crystal growthtechnique not only includes a technique for reducing the defect densityof a fabricated semiconductor layer uniformly as a whole but also isaimed at examining what is called the selective crystal growth forreducing the defect density only in the desired region.

A second object of the present invention is to lengthen the life time ofthe semiconductor device fabricated using the above-mentioned crystalgrowth technique or to improve the life or mobility of the carriersinvolved in the operation of the semiconductor device to a sufficientvalue for practical application of the semiconductor device. Especiallywith the light-emitting diode or the laser diode, the second object ofthe invention is to define a configuration of an optical waveguidesuitable for geometrically controlling or reducing the optical loss ofthe waveguide and the active layer based on the above-mentionedselective crystal growth method, and a configuration suitable forobtaining a stimulated emission light with high internal quantumefficiency from a flat or smooth active layer of the laser device.Specifically, the technique according to one aspect of the second objectof the invention is aimed at defining the structure of a waveguide andan active layer capable of guiding the fundamental transverse modestably in the wavelength of blue-violet region and suitable forrealizing a laser diode operating with low threshold current and highefficiency.

1. Introduction

One of the present inventors proposed a semiconductor laser device asdescribed below in the specification of JP-A-7-238142 forming thefoundation for the declaration of priority of the present application.An example will be described with reference to FIG. 1. A first nitridesemiconductor layer (including a GaN buffer layer 2 and a n-type GaNoptical waveguide layer 3) is formed by crystal growth on asingle-crystal substrate 1 of sapphire (α-Al₂O₃) having a (0001)Csurface, and then an insulator mask 4 is formed on the first nitridesemiconductor layer. This insulator mask 4 has regular patterns ofrectangular window regions, in which the upper surface (the uppersurface of the n-type GaN optical waveguide layer 3) is exposed. Underthis condition, a second nitride semiconductor layer (a n-type GaNoptical waveguide layer 5) is selectively grown on the insulator 4 (onthe first nitride semiconductor layer in the window regions). FIG. 1B isa plan view of the insulator mask 4, and FIG. 1A is a sectional viewtaken in line A-A′ in FIG. 1B.

Specifically, the semiconductor laser device proposed by one of thepresent inventors has a feature in that a nucleation region for crystalgrowth of the second nitride semiconductor layer is confined to thesurface of the n-type GaN optical waveguide layer 3 exposed in theabove-mentioned window regions, and by thus improving thethree-dimensional growth density, the second nitride semiconductor layeris grown in such a manner as to fill the window regions first and whenthe uppermost surface of the grown portions reaches a level flush withthe upper surface of the insulator 4, the growth of the second nitridesemiconductor layer is started on the insulator 4. The surface of eachportion of the second nitride semiconductor layer protruded from eachwindow region grows to extend in the directions parallel andperpendicular to the upper surface of the insulator 4. In the case wherethe window areas of substantially the same size are arranged regularly(or substantially equidistantly), therefore, the crystal layers thathave extended from each pair of adjacent window regions coalesce witheach other on the insulator 4 substantially at the same time. Thisproduces the effect of reducing the crystal defects or crystal grainboundaries of the second nitride semiconductor layer, as compared withthe normal bulk growth in which the second nitride semiconductor layeris formed on the insulator 4 as a crystal layer having a flat surface ofgrowth without using any insulator mask.

The present inventors fabricated several lots of the above-describedsemiconductor laser devices according to the present invention on aninsulator of SiO₂, and by reducing the thickness of these lots along thedirection of crystal growth, observed them under the transmissionelectron microscope (TEM). The semiconductor laser devices each wasfabricated by forming a first nitride semiconductor layer, an insulatorhaving openings (window regions) and a second nitride semiconductorlayer, in that order, on the (0001)C surface of a sapphire (α-Al₂O₃)single-crystal substrate. Also, a portion of the second nitridesemiconductor layer is formed in the openings, and coupled to the firstnitride semiconductor layer on the bottom of the openings. The followingknowledge has been obtained from what is called the cross-sectional TEMimage of this semiconductor laser device.

Knowledge 1: The crystal defect density of the second nitridesemiconductor layer grown on the SiO₂ film (insulator) is in or lowerthan the range of 10⁴ to 10⁵ cm⁻². In contrast, the crystal defectdensity of the second nitride semiconductor layer grown from the uppersurface of the first nitride semiconductor layer in the openings of SiO₂is at the same level of 10⁹ to 10¹¹ cm⁻² as reported in the past. Mostof the defects (dislocations) observed in the second nitridesemiconductor layer grown in the openings are originated in theinterface of the sapphire substrate and intrude into the openingsthrough the first nitride semiconductor layer, while only a smallportion of the defects intrudes into the second nitride semiconductorlayer formed on the SiO₂ layer. In other words, the defects observed inthe second nitride semiconductor layer on SiO₂ sharply decrease awayfrom the SiO₂ openings. Origination of the defects is shownschematically by dashed lines in FIG. 1.

Applying Knowledge 1 to the unit structure of the hexagonal crystalsystem shown in FIG. 2, the present inventors confined the nucleationfor crystal growth of a nitride semiconductor crystal by formingopenings in an insulator, and with regard to a nitride semiconductorlaser device formed with an increased nucleation density, the inventorshave examined as follows:

The growth of the crystal having a hexagonal zinc sulfide (wurtzite)structure which is a kind of a hexagonal crystal system has such afeature that defects extend selectively along c-axis but do not multiplyalong other axes. A transmission electron diffraction pattern hasconfirmed that the second nitride semiconductor layers grown in theopenings and on the insulator are both the crystal of wurtzitestructure. The nitride semiconductor layer on the insulator, however, issubstantially free of the defects extending along c-axis constitutingthe feature of the wurtzite structure. Suppose that the second nitridesemiconductor layer is divided into two regions about imaginaryinterfaces (indicated by one-dot chains in FIGS. 1A and 1B) extendingalong the side walls of the openings. Almost all the defects in theregions on the insulator are successors to the defects formed in theopenings and are received at the imaginary interfaces. Consequently, onthe assumption that the region on the insulator grows in the directionsubstantially perpendicular to c-axis from the imaginary interfaces(i.e., in what is called homoepitaxial growth), the low defect densityin the particular region is considered due to the property of crystalgrowth of the wurtzite structure in which no defect multiplies alongother than c-axis direction.

In view of this, the present inventors have concluded as follows:

Conclusion 1: The crystal of a nitride semiconductor grows extending invertical direction from the surface of the region having a crystalstructure in the openings of the insulator (i.e., in the directionperpendicular to the particular surface) following the atomicarrangement of the same surface, and the nitride semiconductor grown andprotruded out of the openings extends onto the insulator in transversaldirection (i.e., in the direction substantially parallel to the uppersurface of the insulator) from the sides of the nitride semiconductorprotruded from the openings as new growth interfaces. In other words,the crystal growth of a nitride semiconductor is what is called theselective crystal growth exhibiting a behavior in the openings differentfrom that on the surface of the insulator. In the latter case, i.e., onthe surface of the insulator, the crystal growth is substantiallyhomoepitaxial.

The c-axis is an axis of the coordinates (called the crystal axis) fordefining the atomic arrangement of the unit cells of the crystal of thehexagonal symmetry structure. In FIG. 2, the c-axis is designated byarrow (unit vector) c (a₁ axis, a₂ axis and a₃ axis are also similarlydesignated). In FIG. 2, group-III atoms (Ga, Al, etc.) are designated bywhite circles, and group-V atoms (N, As, etc.) by black circles. Thecrystal surface of the hexagonal symmetry structure is expressed byindex (a₁, a₂, a₃, c) defined by these unit vectors, an example of whichis shown in FIG. 2. Sapphire has also a crystal structure of thehexagonal system like a nitride semiconductor, and therefore the (0001)Csurface thereof is a crystal surface orthogonal to c-axis as obviousfrom FIG. 2. The a₁-axis and a₂-axis in the crystal structure of thehexagonal symmetry structure are alternatively expressed as the a-axisand the b-axis, respectively. On the basis of this notation, the a-axis,b-axis and the c-axis are sometimes expressed by the index [a, b, c]which normally represents [100], [010] and [001]. The fundamentals ofthe crystal structure of the hexagonal system were described above. Withreference to FIG. 2, it will be understood that the above-mentionedfirst nitride semiconductor layer epitaxially grows along c-axis on the(0001) plane of the sapphire crystal substrate which is also thehexagonal symmetry structure. (The crystal of the first nitridesemiconductor layer grown in this way is called to be “oriented alongc-axis” with respect to the sapphire substrate).

For in-depth examination of the truth of the selective growth of thenitride semiconductor based on Conclusion 1, the present inventors haveexamined the crystalline property of the crystal structure (amorphous orsingle crystal) and the component elements of the insulator having theopenings and the crystalline property of the nitride semiconductorformed in a manner to cover the insulator. In conducting thisexamination, as shown in FIGS. 3A to 3E, an insulator 4 having anopening 40 is formed or bonded on the (0001)C surface of a sapphiresubstrate 1, and like in the semiconductor laser device described above,a nitride semiconductor crystal 5 was grown on the insulator whileconfining the nucleation region to the opening. As the result of varyingcompositions of the insulator and the nitride semiconductor by lots, thefollowing knowledge were obtained:

Knowledge 2: In the case where the insulator is made of an amorphousmaterial such as SiO₂, Si₃N₄ (SiN_(x)), SiO₂:P₂O₅ (PSG), SiON or Ta₂O₅,the defects in the nitride semiconductor layer formed on the insulatorsharply decrease with the distance away from the opening. Also, thecrystal defect density in the nitride semiconductor layer formed on theinsulator is in or lower than the range of 10⁴ to 10⁵ cm².

Knowledge 3: In the case where the insulator is made of SiC or BaTiO₃having a crystal structure, on the other hand, the growth of a nitridesemiconductor started in the opening (on the sapphire substrate) at thesame time as on the insulator. The grown surfaces of the nitridesemiconductors had unevenesses reflecting the presence or absence of awindow region, and crystal defects developed on the insulator in adensity (in the range of 10⁸ to 10¹¹ cm⁻²) comparable to that in theopening.

Knowledge 4: Further, in the case where the surface of the sapphire wasnon-crystallized partially by irradiating Ga ions and a nitridesemiconductor layer was grown on the non-crystallized surface, crystalhas begun to grow on the non-crystallized surface almost at the sametime as on the surface maintaining a crystal structure. It was foundthat the crystal defect density of the nitride semiconductor layerformed on the non-crystallized surface is lower than that on the surfacehaving a crystal structure, but inferior (in the range of 10⁶ to 10⁸cm⁻²) to the crystal defect density obtained in the nitridesemiconductor layer formed on the SiO₂ layer.

In each of the foregoing experiments, the density defect in the nitridesemiconductor layer grown in the opening was in the range of 10⁸ to 10¹¹cm⁻². These experiments will be described in more detail later withreference to embodiments of the invention.

The present inventors reached the following conclusions from Knowledge 2to 4:

Conclusion 2: In the case where the insulator forming the base of anitride semiconductor is amorphous, the growth of the crystal of thenitride semiconductor becomes more active in the direction perpendicularto what is called the c-axis along which defects are not liable toextend easily. In the case where the insulator has a crystal structure,on the other hand, the crystal of the nitride semiconductor growsheteroepitaxially as defined by the atomic arrangement in the surface ofthe insulator. In other words, for the defects of a nitridesemiconductor crystal to be reduced, it is indispensable to remove theeffects that the atomic arrangement of an insulator may have on theatomic arrangement in the surface of the particular insulator, andtherefore the insulator is required to have an amorphous structure.

Further, in order to verify Conclusion 2, the present inventors attacheda minuscule droplet of Ga atoms to the center of the amorphous surfaceof an insulator formed uniformly and substantially flatly, and heatedthe resulting assembly to the growth temperature of 1030° C. in theammonia environment. This experiment will be described in more detaillater in the section of “Introduction” to embodiments of the invention.For the time being, only the result of the experiment will be described.This experiment, in order to clarify the mechanism of crystal growth onan insulator, oxidized the surface of a silicon single-crystal substrateand formed an amorphous SiO₂ film 101. A nitride semiconductor was grownon this SiO₂ film 101 without forming any openings therein. Thefollowing knowledge was obtained from this experiment:

Knowledge 5: A microcrystal having a mirror surface was formed in theregion formed with the droplet of Ga atoms. Further, in the case wherethe temperature of the amorphous SiO₃ film was held at the growthtemperature of 1030° C. and a trimethyl gallium (TMG) gas was suppliedin the ammonia environment, a single crystal in the shape of hexagonalcolumn was gradually grown about the above-mentioned microcrystal. Anobservation of the section of this single crystal under TEM shows thatthe density of the defects found in the crystal is considerably lowerthan the value (10⁴ to 10⁵ cm⁻²) for the above-mentioned selectivegrowth. In fact, some lots were regarded substantially free of defects.

On the basis of the above-mentioned experiment and Knowledge 5 obtainedtherefrom, the present inventors interpreted the crystal growthmechanism for the nitride semiconductor on an insulating materialaccording to the model shown in FIGS. 4A to 4E. In the model of FIGS. 4Ato 4E, Ga atoms (group-III elements) are designated by white circles,and N atoms (group-V elements) are designated by black circles. Also,the direction of movement of each atom was indicated by an arrowattached to each circle. Opinions of the present inventors will bedescribed in detail below.

According to the interpretation of the present inventors, the N atomsand the Ga atoms on the amorphous insulator move about actively insearch of a stable state. This is similar to the behavior of Si atomsforming terraces and atomic steps in the topmost surface of silicon,known as the atomic migration. In the experiment under consideration, Gaatoms were fixed as a droplet (FIG. 4A) on an amorphous insulator.Exposure to an ammonia environment under this condition causes the Natoms supplied from the environment to attach on the insulator and reachthe droplet of Ga atoms. As described above, N atoms of the group-Velement, which share the electrons in the outermost cell with Ga atomsof the group-III element, form a pair in a stoichiometric ratio ofGa:N=1:1. In this way, a stability is attained by forming a compound(FIG. 4B).

Further, the insulator is set at an optimum temperature for growing asingle crystal of GaN. Then, the N atoms concentrated in the Ga dropletbuild up a mutually regular arrangement in order to improve thestability of the compound, thereby forming the above-mentionedmicrocrystal (FIG. 4C). Under this condition, extraneous N atoms notparticipating in the formation of a GaN crystal exist on the insulator.Supplying TMG in this state, the ratio between the number of Ga atomsand the number of N atoms existing on the insulator approaches theabove-mentioned stoichiometric ratio. In view of the fact that both Gaatoms and N atoms move about rapidly on the insulator, however, theprobability of the two types of atoms bumping each other is negligiblylow as compared with the probability of their bumping the GaN crystalfixed on the insulator. Thus, all of the N atoms and Ga atoms suppliedfrom the TMG and the ammonia environment substantially participate inthe growth of the GaN crystal (FIG. 4D). As a result, the crystal growthproceeds over the entire crystal surface of the Ga microcrystal, so thatthe GaN single crystal multiplies while maintaining the shape of ahexagonal column (FIG. 4E). As obvious from the above-mentioned process,the direct growth of a nitride semiconductor crystal on an insulatorstarts with the Ga atoms supplied as a droplet on the insulator as anucleus.

From the foregoing interpretation, the present inventors have obtainedthe following conclusion:

Conclusion 3: A nitride semiconductor layer can be grown directly on aninsulator of an amorphous structure without depending on the selectivecrystal growth described above. In this case, it is necessary to form anucleus for crystal growth on the insulator. This nucleus issufficiently composed of atoms of a group-III element alone.

Conclusion 3 indicates that no new crystal growth occurs unless anucleus for crystal growth is supplied on the insulating layer on theone hand and that an unexpected crystal growth occurs depending on thetype of atoms existing in the surface of an insulator having anamorphous structure on the other. Specifically, according to Conclusion3, it is possible to explain consistently, as described below, that anitride semiconductor layer begins to grow on the non-crystallizedsurface of a sapphire substrate obtained as Knowledge 4 almost at thesame time as on the surface maintaining a crystal structure, and thatthe crystal defect density of such a nitride semiconductor layer ishigher than that of the nitride semiconductor layer grown on a SiO₃film.

First, the substantially simultaneous crystal growth in thenon-crystallized portion and the crystal portion is caused by the factthat the Al atoms constituting a component element of the sapphireexisting in the non-crystallized surface form a nucleus of a nitridesemiconductor layer as a group-III element. In other words, the Al atomsplay the same role as the droplet of Ga atoms described in Knowledge 5.Consequently, micro-crystals are formed in an irregular arrangement onthe non-crystallized surface of the sapphire constituting an insulatorof an amorphous structure, and the individual single-crystal regionsgrown therefrom coalesce with each other discretely during the crystalgrowth time, thereby giving rise to an unexpected mutual stress betweenthe single-crystal regions. Especially, slight ups-and-downs of thesurface of the insulator presents itself as a difference in theorientation angle of c-axis between the single crystal regions. Thus,the crystal in one region grows in such a direction as to bite into thecrystal in another region, with the result that a stress which inducescrystal defects is generated between the regions involved. The nitridesemiconductor layer on the non-crystallized portion thus develops agreat number of crystal defects, though not as much as experienced bythe nitride semiconductor layer on the crystal portion (i.e, the portionaffected by lattice mismatching).

The above-mentioned experiment revealed the fact that in growing anitride semiconductor layer directly on an amorphous insulator, it iscritical to control the distribution of the group-III elements on thesurface of the insulator, and it became apparent that the insulator isdesirably formed of a material not containing any group-III element as aconstituent element. In other words, the composition of the materialconstituting the amorphous insulator is preferably different from thatof the semiconductor layer formed on the amorphous insulator. Further,it is desirable not to contain any group-III element as a constituentelement. Although the foregoing examination concerns a nitridesemiconductor, i.e., what is called a III-V compound semiconductor whichis composed of at least one of the group-III elements including B, Al,Ga, In and Tl on the one hand and at least one of the group-V elementsincluding N, P, As, Sb and Bi on the other hand and which contains N(nitrogen) as a group-V element, the inventors have concluded that thesame result of examination can be fed back for use with a semiconductorcrystal constituting what is called a III-V compound having a structureof the hexagonal symmetry structure.

Based on the above-mentioned result of examination, the presentinventors propose below a semiconductor material having a newconfiguration and a method of fabrication thereof. The semiconductormaterial referred to herein is not limited to those employed in thestructure of a semiconductor device but includes, for example, a body onwhich a semiconductor device is formed.

Semiconductor material 1: This material comprises a first region made ofa crystal of a compound semiconductor containing at least nitrogen as aconstituent element and a second region made of an insulator, wherein atleast a portion of the first region is grown on the second region.

Semiconductor material 2: This material comprises a first region made ofa compound semiconductor having a crystal structure of the hexagonalsymmetry structure and a second region made of an insulator having anamorphous structure, wherein at least a portion of the first region isgrown on the second region.

The above-mentioned semiconductor materials include those characterizedin that the density of defects existing in the crystal of the portion ofthe first region grown on the second region is not more than 10⁷ cm⁻².The above-mentioned semiconductor materials also include thosecharacterized in that the compound semiconductor making up the firstregion is composed of a group-III element and a group-V element.

Semiconductor material fabrication method 1: This method ischaracterized by comprising a step of growing the crystal of a compoundsemiconductor containing at least nitrogen as a constituent element onthe surface of an insulator having an amorphous structure.

Semiconductor material fabrication method 2: This method ischaracterized by comprising a step of growing the crystal structure of ahexagonal symmetry structure of a compound semiconductor configured of agroup-III element and a group-V element on the surface of an insulatorhaving an amorphous structure.

The above-mentioned methods of fabricating a semiconductor material, inwhich an insulator is formed on a crystal substrate of the hexagonalsystem and has at least an opening.

The above-mentioned semiconductor materials and methods for fabricationthereof are promising as a technique for providing a nitridesemiconductor now closely watched as a material of a light-emittingdiode for emitting the light of wavelengths from green to ultra-violetray, i.e. a material with low defect density having a crystal structureof the hexagonal system composed of at least one of the group-IIIelements (especially, Ga, Al and In) and the N (nitrogen) element.

2. Application to Semiconductor Devices

The inventors propose a configuration of a semiconductor device realizedby the above-mentioned fabrication technique of a semiconductormaterial. Specifically, the present invention provides a semiconductordevice fabricated by forming a conventional nitride semiconductor byheteroepitaxial growth, in which the structural problem of theconventional semiconductor device unavoidably caused by theheteroepitaxial growth is avoided by combining the above-mentionedtechnique of transverse homoepitaxial growth.

The present invention will be briefly explained with reference to anapplication to a semiconductor optical device (a general term for asemiconductor laser device, an optical modulator and an optical switch)as an example of a semiconductor device. The significant basic featureof the semiconductor optical device according to this invention is thata semiconductor layer constituting an optical crystal region (a generalterm indicating regions for emitting, absorbing, containing or guidingthe light, including an active layer and an optical waveguide) is formedon an amorphous insulator. Specifically, a semiconductor layer making upan optical crystal region or a semiconductor layer forming the basethereof is formed by the transverse homoepitaxial growth techniquedescribed above thereby to reduce the density of the defects generatedin the crystal layer. From the viewpoint of the process for fabricatingthe device, unlike in the prior art for forming an optical crystalregion by repeating the heteroepitaxial growth on the main surface of asubstrate, the invention is characterized in that an insulator having atleast an opening is formed on the main surface of a substrate and asemiconductor layer is formed by homoepitaxial growth on the insulator,after which an optical crystal region is formed as a semiconductor layerin or on the homoepitaxial layer by heteroepitaxial growth.

As described above, in the transverse crystal growth of a nitridesemiconductor occurring on an region composed of an insulator having anamorphous structure (such as a body of an insulator), the semiconductorcrystals grown transversely from different regions are transverselycoalesced on the insulator region or on the body. This insulator regionis formed as a mask-like insulator having at least an opening, and thusthe growth of the crystal of a nitride semiconductor layer on theinsulator is controlled, thereby considerably reducing the crystaldefects such as dislocations in the nitride semiconductor layer formedon the insulator. With the semiconductor optical device according to thepresent invention characterized in that an optical crystal region isformed in a nitride semiconductor layer formed on the insulator or in asemiconductor layer formed by epitaxial growth on the nitridesemiconductor layer, the crystal defect density in the optical regioncan be suppressed within or lower than the range of 10⁴ to 10⁵ cm⁻².With the conventional semiconductor optical device with an opticalcrystal region formed by sequential heteroepitaxial growth of thecrystal of a nitride semiconductor on a crystal substrate (such as asapphire substrate) having a different lattice constant, on the otherhand, the crystal defect density occurring in the optical crystal regionis 10⁸ to 10¹¹ cm⁻².

The reduction of the crystal defects in the optical crystal regionaccording to the present invention obviates all the problems of thescattering loss of light and the shortened life of the carrierscontributing to light emission due to crystal defects at a time.Especially, the reduced crystal defects of an optical waveguidesuppresses the loss of optical gain due to the scattering in theresonative amplification of stimulated emission light and thereforesecures an operation with low threshold current and high efficiency.

Also, if a semiconductor layer between an optical crystal region and anelectrode (hereinafter referred to as the contact layer) is formed byhomoepitaxial growth on an insulator, an increased amount of n-type orp-type impurities can be doped into the contact layer. As a result,carriers can be easily generated by doping impurities into the contactlayer, thereby making it possible to set the concentration of n- andp-type carriers to a high value on the order of 10¹⁸ to 10¹⁹ cm⁻³.Consequently, in an application to a semiconductor laser device, forexample, carriers of high density can be injected toward the opticalactive layer from the optical waveguide layer due to the reducedresistance of the contact layer. Thus, the optical gain can be improvedand hence an operation can be performed with low threshold current andhigh efficiency.

The use of the waveguide structure described below is effective forimproving the effect of a semiconductor optical device according to theinvention. Well-known waveguide structures include a gain-guidedstructure with a limited region of an optical active layer for securinga gain by a current-blocking layer, and a refractive index-guidedstructure having a refractive index difference transversely of anoptical active layer by such a stripe structure as a ridge stripestructure or a buried heterostructure (BH). Among these structures, therefractive index-guided structure, which can guide the wave stably infundamental transverse mode, is especially important for assuring anoperation with low threshold current and high efficiency. According tothis invention, a substantial optical crystal region defined by thewaveguide (i.e., the portion at which carriers are injected or where anelectric field is applied) is set on an insulator so that carriers areinjected and the electric field is applied intensively into an opticalactive layer and an optical waveguide layer composed of a semiconductorcrystal of low defect density, thereby improving the emission efficiencywith respect to the amount of injected carriers and the opticalmodulation efficiency according to the electric field intensity. Thewaveguide structure described above can be formed on, under or in theoptical crystal region (in an optical guide layer or in a claddinglayer). As a result, a refractive index-guided structure for propagatingthe laser beam stably in fundamental transverse mode can be realizedwhile at the same time forming a waveguide structure of a crystal layerlow in defect density and high in quality. A semiconductor laseroperating with low threshold current and high efficiency can thus beproduced. Also, a semiconductor laser device in a stripe extended in thedirection of laser cavity is recommended, the shape of which is not ofcourse limited. Further, a striped current-blocking layer and a layerfor forming a refractive index difference, which can be formed byetching a semiconductor layer, can alternatively be formed newly as aninsulator having at least an opening. In such a case, the refractiveindex-guided structure can be easily realized by selectively growing asemiconductor layer using an insulator having a stripe pattern as amask.

Furthermore, in order to improve the crystallinity of the semiconductorlayer constituting an optical crystal region, it is recommended thatdummies be formed by selective crystal growth in such positions as tosandwich the optical crystal region. For the purpose of configuring awaveguide layer composed of a crystal layer of higher quality, forexample, an insulator is used which has a dummy pattern on each side ofa pattern (opening) for producing a waveguide structure by selectivegrowth. By doing so, the unusual growth is avoided in forming awaveguide at the central portion and thus the crystallinity and thegeometric controllability of the particular waveguide are remarkablyimproved. Also, the threshold current and the operating efficiency ofthe device are further improved by a configuration in which current isprevented from being injected from the electrode formed on the waveguideinto the crystal layers grown in the dummy pattern.

The single-crystal substrate used for fabricating a semiconductoroptical device according to the present invention is not limited to theabove-mentioned sapphire substrate. The points to be noted in using anew single-crystal substrate in place of the sapphire substrate aredescribed below. In the case of using a single-crystal substrate ofhexagonal wurtzite structure, for example, the orientation of thesubstrate surface is set to (0001)C plane. When fabricating a stripestructure on this substrate, the insulator mask pattern is set in thedirection perpendicular or parallel to the (1120)A plane of thesubstrate. In this way, individual waveguide crystal layers having arectangular section can be coalesced, thereby making it possible tofabricate a single large waveguide structure easily by use of theselective growth technique. In the case of using a single-crystalsubstrate of a cubic zinc-blende structure, on the other hand, theorientation of the substrate surface is set to the (111) plane. Whenfabricating a stripe structure on this substrate, an insulator maskpattern is set in the direction perpendicular to the (110) plane orperpendicular to the (1-10) plane of the substrate. A waveguidestructure having a crystal in the shape similar to the above-mentionedcase can thus be fabricated.

Various types of semiconductor optical devices according to theinvention can be realized by employing various shapes of the openingsformed in the insulator constituting the base for homoepitaxial growth.First, by quantizing the width of the openings of the insulatorone-dimensionally or two-dimensionally transversely of an active layer,an optical active layer is produced with a quantum box structure orquantum wire advantageous for the operation of the laser device with lowthreshold current. Also, if a plurality of parallel striped openings areformed and the phase-matching conditions are appropriately regulated forthe light generated from each semiconductor layer (active layer)selectively grown between the stripes, then a semiconductor device of aphased array structure can be configured thereby to achieve ahigh-output operation in fundamental transverse mode.

A semiconductor device according to the invention was described abovetaking a semiconductor optical device as an example. Nevertheless, theoptical crystal region can be replaced with a region in which switchablecarriers flow, thereby constituting a field effect transistor, forexample. In the case of a field effect transistor, a semiconductor layermaking up what is called a channel for activating the carriers isdesirably formed on an insulator. An even higher effect is produced ifthe source region, the gate region, the drain region, etc. for thischannel are formed on the insulator. The feature of an example of apreferable device configuration is that a source electrode, a gateelectrode and a drain electrode are formed in juxtaposition on a uniform(openingless) amorphous and insulative region through a semiconductorregion constituting a channel. This semiconductor device configurationis employed especially effectively for a high electron mobilitytransistor (HEMT) easily affected by the crystal defect density of thechannel region.

Based on the above-mentioned result of examination, the presentinventors propose below a semiconductor device having a newconfiguration. The term “semiconductor region” herein, unless otherwisespecified, refers to the one formed of a nitride semiconductor materialor a compound semiconductor material having a hexagonal crystalstructure. Both semiconductor materials were defined in detail in“Introduction”. Also, the “insulative region” is defined as a body or alayer (film) made of a material having an amorphous structure andexhibiting an electrical insulation property, and unless otherwisespecified, is assumed not to be formed with any opening (i.e. a regionwhere another region having a crystal structure is exposed). Anymaterial meeting these conditions can be used to form an insulativeregion.

Semiconductor device 1: This semiconductor device is formed with asemiconductor region making up an optical system on an insulativeregion. The optical system represents the above-mentioned opticalcrystal region. With a laser device, a cavity structure for lasing isdesirably arranged on the insulative region. This semiconductor deviceconfiguration can be employed for all of what are called semiconductoroptical devices including a light-emitting diode (LED), a lighttransmission path and an optical modulator as well as for asemiconductor laser device.

Semiconductor device 2: This semiconductor device is formed with asemiconductor region composed of semiconductor layers having differentband gaps (energy gaps) on an insulative region. These semiconductorlayers constituting the semiconductor region include a firstsemiconductor layer and second semiconductor layers formed on and underthe first semiconductor layer and having a larger band gap than thefirst semiconductor layer. The first semiconductor layer is used forinjecting, confining or generating carriers. The second semiconductorlayers, on the other hand, assist the first semiconductor layer ininjecting or confining carriers. This semiconductor region can be whatis called a quantum well structure with the thickness of the firstsemiconductor layer not more than the de Broglie wavelength or amultiple quantum well structure with the first and second semiconductorlayers alternately formed in multiple stages. This device configurationis applicable also to a field effect transistor, a switching device anda logically operating device as well as to a semiconductor opticaldevice. Also, the second semiconductor layers on and under the firstsemiconductor layer can have different compositions or different bandgaps. This configuration is effective for realizing a field effecttransistor, in which case the second semiconductor layer far from thegate electrode can be done without.

When any one of the above-mentioned semiconductor devices is fabricatedin an existing semiconductor equipment, the above-mentioned insulativeregion can be formed on a region having a crystal structure, i.e., on acrystal substrate or a crystal layer (film).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram showing a longitudinal section (section taken inline A-A′ in FIG. 1B) of an device structure of a semiconductor laserdevice according to the present invention.

FIG. 1B is a diagram showing the upper surface of an insulator of thesemiconductor laser device of in FIG. 1A.

FIG. 2 is a diagram showing a unit crystal structure of a hexagonalsymmetry structure.

FIGS. 3A to 3E are diagrams chronologically showing the crystal growthby the crystal growth technique 1 according to the invention.

FIGS. 4A to 4E are diagrams chronologically showing the growth of a GaNcrystal on an amorphous insulator according to the basic concept of theinvention.

FIGS. 5A to 5D are diagrams chronologically showing the crystal growthby the crystal growth technique 2 according to the invention.

FIG. 6A is a diagram for explaining a mass production process ofsemiconductor devices to which the crystal growth technique according tothe invention is applied.

FIG. 6B is a longitudinal sectional view of an example of a completedsemiconductor device.

FIG. 6C is a perspective view of another example of a completedsemiconductor device.

FIG. 7A is a diagram showing an example of a longitudinal section of asemiconductor optical device according to a first embodiment of theinvention.

FIG. 7B is a top plan view of an insulator mask formed in the deviceshown in FIG. 7A.

FIG. 7C is a longitudinal sectional view of another example of a device.

FIG. 8A is a longitudinal sectional view of an example of asemiconductor optical device according to a second embodiment of theinvention.

FIG. 8B is a top plan view of an insulator mask formed in the deviceshown in FIG. 8A.

FIG. 8C is a longitudinal sectional view of another example of a device.

FIG. 9A is a longitudinal sectional view of an example of asemiconductor optical device according to a third embodiment of theinvention.

FIG. 9B is a top plan view of an insulator mask formed in the deviceshown in FIG. 9A.

FIG. 9C is a top plan view showing another example of an insulator mask.

FIG. 10A is a longitudinal sectional view of an example of asemiconductor optical device according to a fourth embodiment of theinvention.

FIG. 10B is a top plan view of an insulator mask formed in the deviceshown in FIG. 10A.

FIG. 11A is a longitudinal sectional view of an example of asemiconductor optical device according to a fifth embodiment of theinvention.

FIG. 11B is a top plan view of an insulator mask formed in the deviceshown in FIG. 11A.

FIG. 12A is a longitudinal sectional view of an example of asemiconductor optical device according to a seventh embodiment of theinvention.

FIG. 12B is a top plan view of an insulator mask formed in the deviceshown in FIG. 12A.

FIG. 13A is a longitudinal sectional view of an example of asemiconductor optical device according to an eighth embodiment of theinvention.

FIG. 13B is a top plan view of an insulator mask formed in the deviceshown in FIG. 13A.

FIG. 13C is a longitudinal sectional view of another example of adevice.

FIG. 14A is a longitudinal sectional view of an example of asemiconductor optical device according to a ninth embodiment of theinvention.

FIG. 14B is a top plan view of an insulator mask formed in a device.

FIG. 15A is a longitudinal sectional view of an example of asemiconductor optical device according to a tenth embodiment of theinvention.

FIG. 15B is a top plan view of an insulator mask formed in the deviceshown in FIG. 15A.

FIG. 15C is a longitudinal sectional view of another example of adevice.

FIG. 16A is a longitudinal sectional view of an example of asemiconductor optical device according to an 11th embodiment of theinvention.

FIG. 16B is a top plan view of an insulator mask formed in the deviceshown in FIG. 16A.

FIG. 16C is a longitudinal sectional view showing another example of adevice.

FIG. 17A is a longitudinal sectional view of an example of asemiconductor optical device according to an 12th embodiment of theinvention.

FIG. 17B is a top plan view of an insulator mask formed in the deviceshown in FIG. 17A.

FIG. 17C is a longitudinal sectional view of another example of adevice.

FIG. 18A is a longitudinal sectional view of an example of asemiconductor optical device according to a 14th embodiment of theinvention.

FIG. 18B is a top plan view of an insulator mask formed in a device.

FIG. 18C is a longitudinal sectional view of another example of adevice.

FIG. 19 is a photograph corresponding to FIG. 3D taken undertransmission electron microscope (TEM).

FIG. 20 is a photograph corresponding to FIG. 5C taken under scanningelectron microscope (SEM).

BEST MODE FOR CARRYING OUT THE INVENTION

1. Introduction

First, explanation will be made about a semiconductor crystal growthtechnique constituting a basic concept of the present invention. Theexamination on the result of experiments on this technique was describedin “Means for solving the problems”. The knowledge about crystal growthobtained through these experiments will be described in detail below.

<Crystal growth technique 1>

First, a crystal growth technique according to the invention usingexisting crystal growth equipments will be explained with reference tothe process flow of FIGS. 3A to 3E. In the first step, a SiO₂ film isformed on the (0001) plane of a sapphire substrate 1, for example. Thisstep is performed by vapor phase growth using a monosilane gas andoxygen, for example. Specifically, a SiO₃ film is formed by vapor phasegrowth directly on the sapphire substrate. Then, a photoresist is coatedon the surface of the SiO₂ film, and the region to be formed withopenings is exposed to light in stripes and thus removed. Finally, theopenings are formed by wet-etching the SiO₂ film using hydrofluoric acid(HF) etching solution (etchant). These steps use a technique normallyemployed for a Si device and no drawings therefor are prepared.

The third and subsequent steps will be explained with reference to FIGS.3A to 3D. First, the sapphire substrate 1 carrying the SiO₂ film 4formed with an opening 40 (hereinafter called the SiO₂ film mask 4) inthe second step is placed in a furnace for nitride semiconductor crystalgrowth. The latest model of this crystal growth furnace is described inJP-A-4-164895 (U.S. patent Ser. No. 5,334,277). The present inventors,however, employed an existing MOCVD equipment using one of raw gassupply lines for supplying ammonia gas. In the third step, the furnacepressure is set to a level approximate to the atmospheric pressure (760Torr). An ammonia (NH₃) gas and a trimethyl gallium (TMG) gas werecontinuously supplied into the furnace at the rate of 2 to 5 liters (2to 5 SLM) per minute and at the rate of about 10 cc per minute (10sccm), respectively. The sapphire substrate set in the furnace washeated to the growth temperature of 1030° C.

FIGS. 3A to 3C chronologically show the growth of a GaN crystalconstituting a nitride semiconductor device in the third step. First,microcrystals 50 of GaN are formed on the surface of the sapphiresubstrate in the opening 40 (FIG. 3A). The GaN microcrystal looks like ahexagonal column erected on the substrate. At this time point, thesapphire substrate is recovered from the furnace and observed under thescanning electron microscope (SEM). It was confirmed that the directionof c-axis is varied from one microcrystal to another. As alreadydescribed, the epitaxial growth of a nitride semiconductor crystalproceeds along c-axis, and therefore different microcrystals 50 grow indifferent directions as indicated by arrows in FIG. 3A.

Consequently, with the progress of growth of microcrystals, the growingsurfaces of crystals come to rub and bite each other. The result is thatalthough a plurality of GaN crystals grown from the microcrystals arecoalesced in the opening, a multiplicity of dislocations are caused bythe stress exerted on the surfaces and internal parts of themicrocrystals thus coalesced (FIG. 3B). The GaN crystals grown this wayin the opening have the crystal structure thereof defined by the atomicarrangement in the heterojunction interface with the sapphire substrate,and therefore is called a heteroepitaxial region 51 for convenience'sake.

When this heteroepitaxial region 51 grows to such an extent as toprotrude out of the opening, the homoepitaxial growth of a GaN crystalbegins along the upper surface of the SiO₂ film mask 4 with each side ofthe heteroepitaxial region 51 along the edge of the opening 40 as a newgrowth interface (FIG. 3C). In other words, the GaN crystal formed onthe upper surface of the SiO₂ film mask has a different growth mechanismfrom the crystals grown in and above the opening. The present inventorswill refer to the regions formed on the SiO₂ film mask 4 ashomoepitaxial regions 52 for convenience' sake. The sectional view ofFIG. 3C, as viewed from the upper surface side of the SiO₂ film mask 4,shows that unevennesses are formed at an angle of about 120° in thegrowth interface of the homoepitaxial region 52 (FIG. 3D). This stemsfrom the fact that the heteroepitaxial region 51 grows in the form ofhexagonal column. The time lag between the growth interfaces (sides ofthe hexagonal column) of the heteroepitaxial region 52 to protrude fromthe edge of the opening onto the upper surface of the SiO₂ film maskconstitutes the difference of the time when the homoepitaxial regions 52begin to grow. This time lag is reflected in the shape of the growthinterface of each homoepitaxial region. An idea for avoiding suchunevennesses of the growth interfaces is forming the opening as ahexagonal column. This idea will be described later in related sections.

The present inventors observed the GaN crystal fabricated in theabove-mentioned steps under electron microscope. It was discovered thatthe density of crystal defects in the homoepitaxial regions 52 isconsiderably lower than that in the heteroepitaxial region 51 asdescribed above. FIG. 19 is a photograph taken under the transmissionelectron microscope and corresponds to FIG. 3D. The line runningvertically at about the center of the photograph represents an edge ofthe opening of the SiO₂ film mask. This photo shows that a multiplicityof defects in the form of streaks exist only on the left side but not onthe right side of the opening edge. Specifically, the density of crystaldefects is in the range of 10⁸ to 10¹¹ cm⁻² for the left region (theheteroepitaxial region 51 in FIG. 3D) and in the range of 10⁴ to 10⁵cm⁻² for the left region (the homoepitaxial regions 52 in FIG. 3D). Thecoalescing of GaN crystals in the process of growth was confirmed alsoin the homoepitaxial region 52. This phenomenon was observed when aplurality of openings were formed as parallel stripes. Even in the casewhere the growth interface has unevennesses as described above, however,the GaN crystals in the homoepitaxial regions were coalesced withoutinducing no substantial crystal defects, and the defect density of thejunction was at most about 10⁶ to 10⁷ cm⁻².

With regard to the above-mentioned new technique of growing crystal fora nitride semiconductor using the SiO₂ film mask 4, the inventorspropose the processes shown in (1) and (2) of FIGS. 6A as a massproduction engineering for A semiconductor device using a nitridesemiconductor. Both of the methods utilize the crystal coalescence ofthe homoepitaxial regions 52. Specifically, SiO₂ film (or amorphousinsulator) masks 4 having a plurality of openings are formed on asubstrate member 1 made of sapphire or a material having a crystalstructure of a hexagonal system thereby to form a multilayer structure55 composed of a nitride semiconductor. FIGS. 6A to 6C show asemiconductor laser device as an example, which can alternatively be anoptical switch or a field effect transistor with equal effect. In short,a region for performing the device operation is formed on thehomoepitaxial region by injecting carriers. For a device having aperformance affected easily by a minor crystal defect, it is desirableto employ the configuration of (2) of FIG. 6A in which a region fordevice operation is formed on other than the upper surface of thecrystal coalescing region on the masks 4. In both (1) and (2) of FIG.6A, the dicing is effected in the direction of arrows after formingelectrodes 10 thereby to produce a discrete semiconductor laser deviceas a unit shown in FIG. 6B. This semiconductor laser device is formed bystacking an optical waveguide layer 63 including a n-typeAl_(0.15)Ga_(0.85)N layer and a n-type cladding layer 62 with n-typeimpurities doped into the homoepitaxial region (GaN layer) formed on themask 4, an active layer 66 including an undoped InGaN multiple quantumwell layer, an optical waveguide layer (not shown) made of p-typeAl_(0.15)Ga_(0.85)N, a capping layer 68 of p-type GaN containingimpurities higher in concentration than the p-type cladding layer 65,and a p-side electrode 10. The carrier injection region in the activelayer 66 thus is limited by a n-type GaN layer 67 (containing impuritiesof the same level as the n-type cladding layer) formed by being buriedin the p-type cladding layer 65. Specifically, in this device, theactive layer under the region not formed with the n-type GaN layer 67participates in the substantive device operation, and therefore thisportion is displaced from the upper portion of the heteroepitaxialregion, as obvious from (1) and (2) of FIG. 6A. In the mass productionengineering shown in (1) and (2) of FIG. 6A, the substrate 1 is lappedand the lower surface of the n-type cladding layer is exposed beforecutting out a discrete device. The n-side electrode 11 can thus beformed on the particular lower surface. The device structure shown inFIG. 6B represents an example fabricated in this manner.

<Crystal Growth Technique 2>

Now, another crystal growth technique based on the present inventionwill be explained with reference to the process flow shown in FIGS. 5Ato 5D. This prototype experiment was conducted in order to introduceKnowledge 5 described above.

First, a Si substrate 100 is thermally oxidized in an oxygen environmentthereby to form an amorphous SiO₂ film 101 on the surface thereof. Then,in the first step, Ga atoms constituting a nucleus for the growth of aGaN film are formed on the SiO₂ film. As a means for executing the firststep, a furnace having a sectional structure as shown in (1) of FIG. 5Ais used. This furnace has a plurality of gas-supplying nozzles arrangedtwo-dimensionally in opposed relation to a holder (not shown) formounting the Si substrate. The gas-supplying nozzles are arranged on oneof two types of gas supply lines. Also, two plates with an aperture areadapted to be inserted between the holder and the gas-supplying nozzles.This furnace having a unique arrangement of gas-supplying nozzles iscalled a furnace with shower head nozzles for convenience' sake.

First, the interior of the furnace with shower head nozzles is purged bya nitrogen gas constituting an inert gas, and after thus setting theinternal pressure of the furnace to about 760 Torr, the Si substrate 100with the surface thereof oxidized is placed therein. Then, two plateswith an aperture are inserted between the Si substrate and thegas-supplying nozzles. In the process, the aperture of the upper plateis set in registry with the nucleation position of the SiO₂ film 101,and the aperture of the lower plate is displaced out of registry withthat of the upper plate. Under this state, a trimethyl gallium (TMG) gasis continuously supplied at the rate of 10 sccm from the upper supplyline while at the same time sliding the lower plate in the directionindicated by arrow in (1) of FIG. 5A, thus instantaneously supplying theTMG gas onto the surface of the SiO₂ film 101. As a result, a nucleus 53composed of a droplet of Ga atoms is formed only at the nucleationposition on the surface of the SiO₂ film 101, thereby substantiallypreventing the Ga atoms from being attached over the entire surface.

In the second step, an ammonia (NH₃) gas is supplied at the rate of 2 to5 SLM from the lower supply line. Then, the two plates with an apertureare removed. At the same time, the Si substrate 100 is heated to about1000° C. by way of the holder (FIG. 5B). By so doing, a GaN crystalbegins to grow about the position formed with the nucleus 53, and acrystal 54 in the form of hexagonal column is formed on the SiO₂ film101 (FIG. 5C). FIG. 20 is a photograph taken under the scanning electronmicroscope (SEM) and corresponds to FIG. 5C. It is seen that the GaNsingle crystal formed on the SiO₂ film 101 is in the shape of hexagonalcolumn.

At this juncture, the inventors propose to suspend the crystal growthbefore the crystal 54 covers the whole surface of the SiO₂ film 101, toetch the surface portion of the SiO₂ film 101 not formed with thecrystal 54 and thereby to form another semiconductor device made of a Simultiple stacked structure. According this technique, a device composedof the multilayer structure 55 of a nitride semiconductor and a drivercircuit 70 for the device can be formed monolithically on the same Sisubstrate 100, as shown in FIG. 6C. A conductive semiconductor layerconstituting the driver circuit 70 is formed in such a manner as to becoupled with the n-type semiconductor layer located at the lower part ofthe multilayer structure 55 (not shown), and a bonding member 71 isformed between the driver circuit 70 and the p-side electrode 10 locatedat the upper part of the multilayer structure 55, thereby realizing ahybrid device such as a semiconductor laser module.

In the second step, the crystal 54 is grown to a largest possible sizeon the surface of the SiO₂ film 101 (within the range not displaced outof the surface). After that, in the third step, the SiO₂ film 101 ismelted by a HF etchant thereby to separate the crystal 54 and the Sisubstrate 100 from each other. As a result, a new substrate membercomposed of GaN single crystal is realized. This substrate member islower in crystal defect density than SiC which is considered as the bestmaterial of a substrate for epitaxial growth of a nitride semiconductor.Consequently, the GaN substrate produced in the third step is superiorto SiC in terms of lattice matching and prevention of dislocation in theprocess of fabricating a nitride semiconductor device.

An example of a semiconductor device formed using this substrate memberis shown in FIG. 6B. The specification of this device is substantiallyidentical to that of the semiconductor laser device described in“Crystal growth technique 1” above. This technique is different,however, in that a n-type cladding layer 62 doped with n-type impuritiesis composed of a GaN substrate doped with n-type impurities. In the casewhere a nitride semiconductor layer is epitaxially grown on a GaNsubstrate in this way, the lattice matching is easily secured betweenthe stacked semiconductor layers. At the same time, even in the case ofconfiguring a pseudomorphic device which exerts a compressive strain anda tensile strain on the semiconductor layer making up an optical crystalregion, the defects due to the introduction of a lattice-mismatchedlayer can be easily suppressed.

The above-mentioned first step can also be executed by applying an ionbeam, for instance. An example will be described with reference to (2)of FIG. 5A. A Si substrate 100 with an amorphous SiO₂ film 101 formed onthe surface thereof is placed on a secondary ion mass spectrometer(SIMS), and Ga ions are irradiated on the surface of the SiO₂ film withan acceleration voltage of 1 kV and a dose of 1×10¹³ ions/cm². The SIMSemployed in this case is generally called a static SIMS which is usedfor measuring the first atomic layer and the adsorbed layer on specimensurface. This SIMS irradiates ions having a current density of 10⁻⁵ to10⁻³ mA/cm² with an energy of 0.5 to 5 keV. Still another type of SIMSis available in which ions with a current density of 1 to 100 mA/cm² areirradiated with an energy of 5 to 30 keV. The latter equipment is usedto sputter the specimen surface by ion irradiation and therefore is notsuitable for forming a nucleus on the SiO₂ film.

For irradiating Ga ions on the surface of the SiO₂ film, the ion beamdiameter was reduced by an ion optical system to 100 Å when reaching thesurface of the SiO₂ film. The ions are irradiated for about a second.The irradiation time was regulated by deflecting the ion beam by a beamdeflector mounted in the ion optical system. After forming the nucleus53 composed of Ga atoms on the surface of the SiO₂ film in this way, theSi substrate 100 was recovered from the SIMS and transferred to thefurnace with shower head nozzles to enter the second step. In theprocess, the furnace interior was purged by nitrogen (N₂) constitutingan inert gas, and the plates with aperture were removed. After placingthe Si substrate in the furnace with shower head nozzles, a trimethylgallium (TMG) gas was supplied from the upper supply line at the rate of10 sccm, and an ammonia (NH₃) gas from the lower supply line at the rateof 2 to 5 SLM. At the same time, the Si substrate 100 was heated toabout 100° C. by way of the holder (FIG. 5B). The subsequent process isidentical to that for the second step.

The above-mentioned method of forming a nucleus with an ion beam usedthe static SIMS. Any other equipment, however, can be used which iscapable of irradiating an ion beam with the same amount of current andthe same amount of energy (acceleration voltage). As compared with amethod using the furnace with shower head nozzles in the first step, themethod of forming a nucleus with an ion beam requires the additionalprocess of transferring the Si substrate from the ion beam irradiationunit to the furnace with shower head nozzles (or a crystal growthfurnace). Nevertheless, the latter method is more advantageous in that anucleation region can be set arbitrarily and in that the plates with anaperture are not required for the furnace with shower head nozzles(i.e., the configuration of the furnace for crystal growth issimplified).

2. Application to Semiconductor Devices

A method of fabricating a semiconductor device using the crystal growthtechnique according to the invention described in “Introduction” abovewill be described in more detail below with reference to embodiments.Each embodiment will be explained using a semiconductor laser deviceconstituting one of semiconductor optical devices as a model.

<Embodiment 1>

A semiconductor device according to an embodiment of the presentinvention will be described with reference to FIGS. 7A and 7B. In FIG.7A, a GaN buffer layer 2 and a n-type GaN optical waveguide layer 3 aregrown as crystal by metal organic vapor phase epitaxy on a sapphire(α-Al₂O₃) single-crystal substrate 1 having a (0001)C plane, forexample. After that, a pattern (of openings) 40 having at least twostriped window regions is formed for the insulator masks 4 as shown inFIG. 7B. In the process, the insulator masks are striped in thedirection parallel to the (11-20)A plane of the sapphire substrate 1.Then, a n-type GaN optical waveguide layer 5 is selectively grown ineach of the two window regions formed by the insulator mask patterns 4.The resulting two layers 5 are coalesced transversely into a flat,rectangular single n-type GaN optical waveguide layer 5 over the centralinsulator mask. Then, a compressive-strained multiple quantum wellactive layer 6 including an AlGaN separate confinement heterostructurelayer, a GaN quantum barrier and a GaInN compressive-strained quantumwell, a p-type GaN optical waveguide layer 7 and a p-type GaInN contactlayer 8 are formed in that order. Next, insulator masks 9 are formed bylithography, and a pattern of the p-side electrode 10 and the n-sideelectrode 11 is deposited by evaporation. Finally, the surface of acavity is cut off by cleaving the assembly in the directionperpendicular to the stripes, and a device is scribed off. In this way,a longitudinal section of the device (section taken in line B-B′ in FIG.7B) shown in FIG. 7A is produced.

According to this embodiment, a gain-guided structure can be produced byregulating the width of each insulator mask and the width of each windowregion and thus by setting the width of the active layer on the opticalwaveguide layer 5 to not less than 5 μm. Also, a refractive index-guidedstructure with a buried heterostructure can be produced by setting thewidth of the active layer in the range of 1 to 3 μm in similar fashion.In this technique, the crystal defect density was in about the samerange of 10⁹ to 10¹¹/cm² as in the conventional method in the windowregions of the insulator masks for selective growth. On the insulatormasks, however, homoepitaxial growth could be realized and the crystaldefect density thereby could be reduced to the range of 10⁴ to 10⁵/cm²or less. In this device structure, the current blocking is effected byan insulators 9. Further, when viewed transversely of the active layer6, the internal optical loss is considerably varied between the centralregion of the active layer 6 and the outer regions of the active layerabove the insulator masks. It follows, therefore, that the laser beam isguided also with a loss thereby to restrict the propagation region. Inthe device having the refractive index-guided structure with a buriedheterostructure according to this embodiment, an operation with lowerthreshold current can be achieved than the device having the gain-guidedstructure. The threshold current can be reduced to ⅓ to ¼ as comparedwith the device having the gain-guided structure. This device alsoperforms the lasing operation with lower threshold current and higherefficiency than the conventional device formed by bulk growth, and has alasing wavelength in the range of 410 to 430 nm at room temperature.

A variation of the present embodiment will be explained with referenceto FIG. 7C (sectional view taken in line B-B′ in FIG. 7B). The processfor fabricating the device and the pattern of the insulator masks 40 aresubstantially identical to those described above. After growing up tothe p-type contact layer 8, however, a ridge stripe is formed on thep-type optical waveguide layer 7 by lithography and etching. In theprocess, the bottom width of the ridge stripe is set to the range of 3to 7 μm. Further, a buried layer 12 is formed on each side of the ridgestripe. This buried layers 12 can be formed either as n-type Gacurrent-blocking layers by selective growth using an insulator ordirectly as a dielectric insulator. The process for forming theinsulators 9 and the electrodes 10, 11 is the same as described above,with the result that the vertical sectional view of the device shown inFIG. 7C is obtained.

The configuration shown in FIG. 7C has a ridge stripe structure forinjecting currents only at the central region of low defect density andlow optical loss in the optical active layer. It is therefore possibleto attain effective current injection and stable refractive indexdistribution. As a result, a threshold current value was obtained equalto or lower than that of the refractive index-guided structure withburied heterostructure shown in FIG. 7A. This device performs the lasingoperation with low threshold current and high efficiency, and the lasingwavelength thereof at room temperature is in the range of 410 to 430 nm.

<Embodiment 2>

A semiconductor device according to another embodiment of the inventionwill be described with reference to FIGS. 8A and 8B. The device isfabricated in a way similar to the first embodiment, except that inaddition to the insulator masks for the waveguide structure according tothe first embodiment, dummy patterns are used to form a pattern(openings 40) of the insulator masks 4 shown in FIG. 8B. The width andinterval of the insulator masks are set in such a way as to prevent thecrystal layers grown on the dummy patterns from being coalesced with thecrystal layer of the waveguide formed at the central portion. Also, theinsulators 9 are covered for preventing a current from flowing in thecrystal layers grown on the dummy pattern (openings on the sides). Inthe remaining points, the same process is performed as in the firstembodiment to produce the vertical sectional view of the device shown inFIG. 8A (the sectional view taken in line C-C′ in FIG. 8C).

According to this embodiment, crystal layers constituting dummies areformed on the waveguide structure of the first embodiment for improvingthe quality and geometry of the crystal layer formed on the centralwaveguide. Unusual growth and unstable growth rate were thus obviatedand a rectangular optical waveguide having flat and smooth upper andside surfaces could be formed without considerably affecting theconditions for crystal growth outside of the dummy pattern. As a result,the laser beam can be guided with lower loss, and the semiconductordevice can be operated with lower threshold current and higherefficiency. It became thus possible to reduce the threshold current toat least ⅔ to ½ as compared with the first embodiment. This deviceperforms the lasing operation in the lasing wavelength of 410 to 430 nmat room temperature.

FIG. 8C is a sectional view (the section taken in line C-C′ in FIG. 8B)of a variation of the present embodiment. This device is fabricatedthrough substantially the same process as the device shown in FIG. 8A.The difference lies, however, in that after forming the layer 8, buriedlayers (such as current-blocking layers) 12 are formed like in thedevice shown in FIG. 7C. Further, insulators 9 are covered in order toprevent a current from flowing in the crystal layers grown on the dummypattern. As compared with the device in FIG. 7C, the device shown inFIG. 8C makes it possible to improve the quality and geometry of thecrystal layer formed on the central waveguide. The threshold current canalso be reduced at least to ⅔ to ½ of the threshold current for thedevice shown in FIG. 7C. This device performs the lasing operation withthe lasing wavelength in the range of 410 to 430 nm at room temperature.

<Embodiment 3>

A semiconductor device according to still another embodiment of theinvention will be described with reference to FIGS. 9A and 9B (FIG. 9Ais a sectional view taken in line D-D′ in FIG. 9B). The device is formedby crystal growth in the same manner as the second embodiment shown inFIG. 8C. The pattern (openings 40) of the insulator mask 4, however, isnot striped as shown in FIG. 9B, but includes a plurality of rectangularopenings (window regions) arranged two-dimensionally at predeterminedintervals. According to this embodiment, as compared with the fourthembodiment, the crystal defect density of the optical active layer 6 atthe central portion of the waveguide could be reduced further. Thestripe structure for guiding the laser beam is formed in the centralregion of the active layer corresponding to the portion on the centralinsulator mask to block the currents. The crystal defect density,however, is desirably small over the entire optical active layer. Forthis purpose, it is effective to increase the proportion of which thearea where homoepitaxial growth is possible represents of the whole areaof the active layer. In fabricating the waveguide structure at thecentral portion, therefore, the proportion of the area occupied by therectangular window regions in the insulator mask for selective growth isreduced as far as possible. This device, as compared with the device ofFIG. 8C, can reduce the crystal defects of the optical active layer andthe optical waveguide layer, and therefore, can assure an operation withstill lower threshold current and still higher efficiency. The thresholdcurrent was reduced to ½ to ⅓ as compared with the device of FIG. 8C.This device performs the lasing operation with the lasing wavelength inthe range of 410 to 430 nm at room temperature.

A variation of this embodiment will be explained with reference to FIG.9C. In this example, the section taken in line D-D′ of the device issimilar to that in FIG. 9A, but the pattern of the openings 40 of theinsulator mask 4 is different. Specifically, a pattern of rectangularopenings is formed on the two sides for dummy crystal growth, whereas apattern of openings in the shape of equilateral hexagon are formed forselective growth corresponding to the central waveguide structure.

The shape of the openings will be briefly described. According to thisembodiment, the openings (also called the window regions) in theinsulating film 4 for exposing the surface of the crystal regionconstituting a base are shaped in quadrangle such as square or rectangleor in hexagon including equilateral hexagon, and arranged atpredetermined regular intervals. As a result, the crystal layers thathave grown out of adjacent openings toward each other by homoepitaxialgrowth on the insulator are easily coalesced with each other on theparticular insulator thereby to form a single optical waveguide or asingle optical active layer. Especially, a nitride semiconductormaterial constituting a III-V semiconductor having a hexagonal symmetrystructure epitaxially grows while maintaining the form of hexagonalcolumn in each opening. Therefore, the interface where eachhomoepitaxial growth starts on the insulator is also formed as a side ofthe hexagonal column.

With the insulator mask 4 shown in FIG. 9C reflecting the property ofgrowth of the hexagonal crystal system, the crystal layers grown in theopenings (window regions) in the shape of equilateral hexagon are easilycoalesced with each other, thereby considerably reducing the chance ofcrystal defects developing at the junctions thereof. As a result, thecrystal defect density of the optical active layer and the opticalwaveguide layer making up the central waveguide structure is reducedbelow that of the device employing the insulator mask of FIG. 9B. Thus,as compared with the insulator mask of FIG. 9B, an operation with lowthreshold current and high efficiency becomes possible with thethreshold current being decreased to the range of ⅔ to ½. This devicealso performs the lasing operation with the lasing wavelength in therange of 410 to 430 nm at room temperature.

<Embodiment 4>

Yet another embodiment of the invention will be described with referenceto FIGS. 10A and 10B. The device is fabricated in a similar manner tothe second embodiment. As shown in FIG. 10B, however, a pattern ofopenings 40 of the insulator masks 4 are arranged in juxtaposition toform a central waveguide structure in an array of stripes. As a result,the central waveguide structure of the second embodiment is formed in aplurality of stripes arranged in parallel thereby to make up a phasedarray waveguide structure. FIG. 10A is a sectional view taken in lineE-E′ in FIG. 10B.

According to this embodiment, the laser beam that has propagated tothree central waveguides can be output in a fundamental mode satisfyingthe phase matching conditions. Thus, an operation of higher output isrealized than in the first to third embodiments, and an output is atleast three to five times as high as the maximum optical output producedin the second embodiment. This device performs the lasing operation withthe lasing wavelength in the range of 410 to 430 nm at room temperature.

<Embodiment 5>

A further embodiment of the present invention will be explained withreference to FIGS. 11A and 11B. This device is fabricated in asubstantially similar manner to the first embodiment. In thisembodiment, however, the first step taken is to form a GaN buffer layer2, an undoped GaN layer 13, a high reflective DBR (Distributed BraggReflector) mirror 14 of undoped GaInN/AlGaN, and a n-type GaN opticalwaveguide layer 3, for example, by crystal growth on the substrate 1.Further, as shown in FIG. 11B, the device includes insulator masks inthe shape of equilateral hexagon and window regions arranged around theperipheries of the insulator masks. Then, after forming up to the layer8 as in the first embodiment, a high reflective DBR mirror 15 of undopedGaInN/AlGaN is grown selectively. The high reflective DBR mirror 15,like the high reflective DBR mirror 14, is formed of a plurality ofGaInN layers and AlGaN layers stacked alternately to constitute amultilayer structure.

In fabricating a semiconductor laser device having a vertical cavitystructure (as defined later) as in the present embodiment, the elementcomposition (Ga:In or Al:Ga) of at least one of the GaInN layer and theAlGaN layer of the high reflective DBR mirrors 14, 15 is preferablychanged thereby to differentiate the overall reflectivity of eachmultilayer structure. In the present embodiment with the upper surfaceof the high reflective DBR mirror 15 as an emission end of the laserbeam, for example, the reflectivity of the high reflective DBR mirror 15is preferably lower than that of the high reflective DBR mirror 14.Also, in the case where the active layer 6 has a strained quantum wellstructure or a strained superlattice structure, the high reflective DBRmirror 14 can also be used for stress compensation of the buffer layer 2and the active layer 6 formed on the substrate.

Let us return to the process for fabricating a semiconductor laserdevice.

After a ridge stripe is formed by etching off the layers 15, 8 and 7,the layer 12 is buried to form the insulator 9. The high reflective DBRmirror 15 located above is formed as a dielectric high reflective DBRmirror and etched as shown in FIG. 11A after forming the insulator 9. Ap-side electrode and a n-side electrode are then deposited byevaporation, and a cavity mirror is fabricated by cleaving in thedirection perpendicular to the stripe. The device is separated by beingscribed there to produce the vertical section of the device shown inFIG. 11A (the section taken in line F-F′ in FIG. 11B).

According to the present embodiment, a waveguide can be formed on thecentral insulator mask by a crystal layer in the shape of equilateralhexagon having a flat and smooth upper surface. An optical active layeris formed on this optical waveguide, thereby producing a vertical cavitystructure for surface emitting (a cavity structure for lasing formed inthe direction substantially perpendicular to the main surface of thesubstrate 1). The semiconductor laser device according to thisembodiment has a functional feature of generating a laser beam in thedirection perpendicular to the main surface of the substrate 1. In thisrespect, the present embodiment is different in configuration from thesemiconductor laser device according to the first to fourth embodimentsdescribed above in which a cavity structure for lasing is formedsubstantially in parallel to the main surface of the substrate 1. Inthis device, the reflectivity of the end surface (the upper surface ofthe high reflective DBR mirror 15) can be set stably at a high value ofnot less than 95 to 99% due to the provision of the high reflective DBRmirror 15. As compared with the devices shown in the first to fourthembodiments (having a laser emitting end surface on the side of theactive layer), the present embodiment can minimize the thresholdcurrent. In fact, the threshold current can be reduced at least to{fraction (1/10)} to {fraction (1/30)} that for the device of the thirdembodiment. The device according to this embodiment performs the lasingoperation with the lasing wavelength in the range of 410 to 430 nm atroom temperature.

<Embodiment 6>

A yet further embodiment of the invention will be explained. Accordingto this embodiment, in place of a sapphire (α-Al₂O₃) substrate, asubstrate 1 is made of n-type silicon carbide (α-SiC) having a hexagonalsymmetry structure with the substrate surface orientation in the (0001)Cplane, and a n-type buffer layer is formed on the substrate of n-typesilicon carbide. The device structure is fabricated in the same processas that of the first to seventh embodiments, thereby producing a sectionof the device.

According to this embodiment, the substrate has a conductivity.Therefore, the substrate can be mounted in such a manner that the n-sideelectrode is deposited by evaporation on the lower surface thereofhaving a junction formed by crystal growth. In this way, current can besupplied to the n-side electrode on the lower surface of the substratefrom the p-side electrode on the upper surface of the substrate througha nitride semiconductor. The heat radiation can thus be remarkablyimproved. This embodiment can produce a laser device which operates at ahigher temperature than the other embodiments. This device performs thelasing operation with the lasing wavelength of 410 to 430 nm at roomtemperature.

<Embodiment 7>

Still another embodiment of the invention will be described withreference to FIGS. 12A and 12B. This embodiment, though geometricallysimilar to the semiconductor device described with reference to thefirst to fourth embodiments, discloses a configuration suitable forsecuring a large width of an active layer.

In FIG. 12A, a GaN buffer layer 2 and a n-type GaN optical waveguidelayer 3 are formed with crystal grown by the metal organic vapor phaseepitaxy on a sapphire (α-Al₂O₃) single-crystal substrate 1 having the(0001)C plane, for example. After that, a pattern of rectangular windowregions 40 of an insulator mask 4 are formed in grid by lithography andetching. In the process, the insulator mask 4 having the window regionsis set with the longitudinal direction thereof perpendicular to the(11-20)A plane of the α-Al₂O₃ substrate 1. Then, a n-type GaN opticalwaveguide layer 5 is formed by continuing selective crystal growth ineach window region until the crystals thus grown are coalescedtransversely into a single flat optical waveguide layer. Immediatelyafter that, a compressive-strained multiple quantum well active layer 6including an AlGaN separate confinement heterostructure layer, a GaNquantum barrier, a GaInN quantum well, and a p-type GaN opticalwaveguide layer 7 are formed in that order. Then, an insulator mask 8 isformed by lithography. Further, a pattern of a p-type electrode 10 and an-type electrode 11 is deposited by evaporation. Finally, a cavitymirror is fabricated by cleaving in the direction perpendicular to thewaveguide stripe, and the device is scribed off thereby to produce alongitudinal section of the device shown in FIG. 12A (the section G-G′in FIG. 12B).

The present embodiment can provide a transversely gain-guided waveguidestructure by increasing the overall width of the insulator mask in gridand setting the optical active layer to not less than 5 μm. On the otherhand, the present embodiment can provide a refractive index-guidedstructure with a buried heterostructure by decreasing the overall widthof the insulator mask and setting the optical active layer in the rangeof 1 to 3 μm. This method improves the density of three-dimensionalnucleation and can promote the transverse coalescence of crystal layersin the first step, thereby making it possible to form the opticalwaveguide 5 having a flat and smooth surface. Also, the continuedselective growth can form an optical active layer and an upper waveguidelayer composed of a crystal layer flat and higher in quality than in themethod employing the bulk growth on the upper flat surface of theoptical waveguide 5. An optical waveguide structure of low optical losscan thus be produced. This device performs the lasing operation withlower threshold current than the device formed by the conventionalmethod of bulk growth, and the lasing wavelength thereof at roomtemperature is in the range of 410 to 430 nm.

<Embodiment 8>

A still further embodiment of the invention will be explained withreference to FIGS. 13A and 13B. The device is fabricated in a similarmanner to the seventh embodiment, but is different in that a pattern ofrectangular window regions (openings) 40 of an insulator mask 4 shown inFIG. 13B are staggered between different columns.

By displacing the pattern of the openings 40 by one half period betweenadjacent columns as in the insulator mask 4 according to the presentinvention, coalescence is promoted between the crystal layershomoepitaxially grown onto the upper surface of the insulator from eachend of adjacent openings, thereby making it possible to form a uniformand flat semiconductor layer. Further, if the openings are miniaturized,a larger width between the window regions can be secured and increasesthe area of the homoepitaxially grown flat layer in the desired shape.

According to this embodiment, it is possible to produce an opticalwaveguide layer 5 having a more flat and smoother upper surface than inthe seventh embodiment. Further, a waveguide structure with a buriedheterostructure can be formed with an upper waveguide layer and anoptical active layer constituting a flat and high-quality crystal layeron the optical waveguide layer 5. As a result, the threshold current canbe reduced to at least one half or less of the threshold current for thefirst embodiment.

A variation of this invention will be explained with reference to FIG.13C. This example is different in that each window region of theinsulator mask 4 is in the shape of equilateral hexagon. Crystal layersin the shape of equilateral hexagon staggered between columns areselectively grown in the window regions in the shape of equilateralhexagon of the insulator mask, and can be easily coalesced with eachother into a flat single optical waveguide layer. Consequently, anoptical waveguide layer 5 has an upper surface more flat and smootherthan that of the device using the insulator mask shown in FIG. 13B.Further, a waveguide structure with a buried heterostructure includingan upper waveguide layer and an optical active layer composed of flat,high-quality crystal layers could be formed on the optical waveguidelayer 5. As a result, the threshold current could be reduced at least toone fourth of the threshold current in the seventh embodiment.

FIG. 13A shows a section taken in line H-H′ in FIG. 13B or 13C.

<Embodiment 9>

A still further embodiment of the invention will be described withreference to FIGS. 14A and 14B. This device is fabricated in a similarfashion to that of the eighth embodiment. As shown in FIG. 14B, however,the columns of openings at the extreme ends of the insulator mask 4 areformed as a dummy pattern for selective growth. The dummy pattern andthe pattern of three central columns of openings are spacedappropriately from each other in order that the crystal layers formed onthem may not coalesce with each other. At the same time, the opticalwaveguide layers 5 on the central pattern of three columns are coalescedinto a single optical waveguide layer, on which a layer 6 is formed as asingle optical active layer structure. The crystal layers grown on thedummy pattern are covered with an insulator not to be supplied with acurrent. In the remaining points, exactly the same process is followedas in the third embodiment. A longitudinal section of the device shownin FIG. 14A (the section taken in line J-J′ in FIG. 14B) is thusobtained.

The present embodiment can produce an optical waveguide layer 5 having amore flat and smoother upper surface than the device of the eighthembodiment employing the insulator mask shown in FIG. 13C. Further, anupper waveguide layer and an optical active layer made of a flat,high-quality crystal layer can be formed on the optical waveguide layer5. As a result, the threshold current can be reduced at least to ⅔ to ½that for the device of the eighth embodiment employing the insulatormask shown in FIG. 13C.

<Embodiment 10>

Another embodiment of the present invention will be explained withreference to FIGS. 15A and 15B. This device is fabricated through asimilar process to the ninth embodiment, except that as shown in FIG.15B, the columns of openings at the extreme ends of the insulator mask 4are formed as a dummy pattern for selective growth. Also, the intervalbetween the columns of the insulator masks is adjusted in such a mannerthat the crystal layers formed on the central pattern of three columnsas optical waveguide layers 5 are not coalesced with each other, so thatthe optical active layers are not coalesced with each other transverselybetween different columns but between the window regions within eachcolumn. As a result, stripes of crystal layers connected in each columnare formed thereby to produce a waveguide structure in phased array. Thecrystal layers grown on the dummy pattern of the columns at extreme endsare covered with an insulator not to be supplied with a current. FIG.15A shows a section taken in line K-K′ in FIG. 15B.

According to this embodiment, an operation in fundamental modesatisfying the phase matching conditions is possible for the waveguidesin the three central columns. Consequently, an operation is realizedwith a higher output than in the seventh to ninth embodiments. An outputis at least three times higher than the maximum optical output obtainedin the ninth embodiment. Also, the pattern width of the insulator maskwas reduced, so that a quantum wire could be formed and arranged inarray transversely by quantization in the direction perpendicular to thestripes.

A variation of this embodiment will be explained with reference to FIG.15C. This device is fabricated in a similar manner to the case of FIG.15A. The only difference lies in that the crystal layers 5 to 7 formedon the central pattern of three columns of openings in the insulatormask 4 are not coalesced with each other. As a result, the crystallayers extending along the length of cavity for lasing (in verticaldirection in FIG. 15B) are never connected to each other. In otherwords, three independent microwaveguides can be formed in parallel. Inthe case where the width of the microwaveguides is reduced somewhat, thecarriers injected into these waveguides are quantized in the directionperpendicular to the length of cavity. Also, the crystal layers grown onthe dummy patterns at the extreme ends are covered with an insulator inorder not to be supplied with current.

In the device shown in FIG. 15C, the waveguide structure of the windowregions making up each of the three central columns includes an opticalwaveguide layer perpendicular to the corresponding substrate surface andforms a multiple cavity mirror. Therefore, the operation in singlelongitudinal mode with small wire width is possible unlike in the deviceof FIG. 15A. Also, the device of FIG. 15C constitutes a phased array andoperates in fundamental mode satisfying the phase-matching conditions.As a consequence, like the device of FIG. 15A, the device of FIG. 15Ccan produce an output at least three times as high as the maximumoptical output of the ninth embodiment. Also, in the case where thepattern width of the insulator mask 4 is reduced (i.e., the area of eachopening 40 is reduced) and the crystal layers 5 to 7 are isolated fromeach other by the insulators 9 in the direction perpendicular to thelength of cavity, then it is possible to produce quantum boxes byquantization in the direction along the cavity length. In this case, andevice can be realized with a plurality of quantum boxes arrangedtwo-dimensionally (in grid) on the substrate 1.

<Embodiment 11>

The configuration of the semiconductor device described with referenceto the embodiments from the present embodiment to the 14th embodiment isbased on the architecture of forming amorphous insulating layers in aplurality of stages on a region (such as a substrate) having a crystalstructure. Each insulating layer is formed with at least an opening, andthe above-mentioned selective growth of a nitride semiconductor isrepeated as many times as the stages of the insulating layers. Theopenings of the insulating layers are desirably staggered in such amanner that the openings of the insulating layer in the x-th stage asviewed from above, for example, is rendered invisible from theinsulating layer in the (x+1)th stage. The object of this architectureis to sharply reduce the crystal defect density in the nitridesemiconductor layer formed on the topmost insulating layer by repeatingsuch a selective growth.

The embodiments including the present embodiment to the 14th embodimentrefer to a semiconductor laser device, in which insulating layers areformed in two stages on a crystal substrate, and an optical crystalregion is formed in the nitride semiconductor layer on the second-stageinsulating layer. In spite of this, the number of stages of insulatinglayers, i.e. the number the selective growth is repeated can be morethan two. Specifically, the number of stages of insulating layers isdetermined according to the trade-off between the quality of the crystalrequired for the optical crystal region or for the region where carriersto be switched flow on the one hand and the production cost dependent onthe number of fabrication steps on the other.

The semiconductor laser devices studied in the present embodiment up tothe 14th embodiment each comprise insulator masks in two stages on asubstrate, and a layer composed of nitride semiconductor crystal isselectively grown in two steps. In this way, the crystal dislocationdensity in the optical crystal region (such as an optical waveguidelayer) can be considerably reduced. Specifically, assuming that thedislocation density of the crystal layer formed by homoepitaxial growthon the insulator mask in the first stage is in the range of 10⁴ to10⁵/cm², for example, the dislocation density of the crystal layerformed by homoepitaxial growth on the insulator mask in the second stageis reduced to the range of 10³ to 10⁴/cm². This crystal dislocationdensity is substantially at the same level as the dislocation density inthe optical crystal region of a semiconductor laser device which alreadyfinds applications, i.e. a device fabricated on a substrate made of aIII-V semiconductor material (such as GaAs or InP) other than thenitride semiconductor. The reduced defect density in the opticalwaveguide layer reduces the loss due to the light scattering or the gainloss due to the carrier trap or the light absorption at deep level, andthus contributes to an operation of the semiconductor laser device withlow threshold current and high efficiency. Consequently, by forming ahigh-quality optical waveguide of low defect density by selective growthin two or more stages, for example, it is possible to realize asemiconductor laser device having a refractive index-guided structurecontrolled in fundamental transverse mode which is adapted to operatewith low threshold current, high efficiency and small internal opticalloss.

Now, a semiconductor laser device according to this embodiment will bedescribed with reference to FIGS. 16A and 16B.

First, insulator masks 4 having two striped openings 40 shown in FIG.16B are formed on the (0001)C plane of a sapphire (α-Al₂O₃)single-crystal substrate 1. The stripes of the insulator masks 4 areformed to extend in parallel to the (11-20)A plane of the sapphiresubstrate 1. Then, GaN buffer layers 22 and a N-type GaN opticalwaveguide layer 23 are formed by selective growth in the first step bythe metal organic vapor phase epitaxy. In the process, the n-type GaNlayer 23 is formed in such a manner that a crystal layer thereof isgrown in each of the two striped openings 40 and the two crystal layersthus formed are coalesced with each other by transverse homoepitaxialgrowth on the central insulator mask thereby to form a single crystallayer.

After that, second-stage insulator masks 41 are formed on the uppersurface of the n-type GaN layer 23. A striped opening 42 is formedbetween the insulator masks 41 over the central insulator mask 4 (i.e.,in the masking region sandwiched between the two striped openings 40).The striped opening 42 extends substantially in parallel to the stripedopenings 40 of the insulator masks 4. Therefore, as viewed from theupper surface of the insulator masks 41, the openings 40 are covered bythe insulator masks 41. Selective growth in the second stage is effectedusing the insulator masks 41 thereby to form a n-type GaN opticalwaveguide layer 5, a compressive-strained multiple quantum well activelayer 6 including an AlGaN separate confinement heterostructure layer, aGaN quantum barrier and a GaInN compressive-strained quantum well, ap-type GaN optical waveguide layer 7 and a p-type GaInN contact layer 8,sequentially in that order. Then, insulators 9 are formed bylithography, and a p-side electrode 10 and n-side electrodes 11 aredeposited by evaporation. Finally, the assembly is cleaved in thedirection perpendicular to the striped insulator masks to fabricate acavity mirror. Then, the device is scribed off thereby to produce alongitudinal section shown in FIG. 16A (the section taken in line L-L′in FIG. 16B). Specifically, this semiconductor device has a cavitystructure which generates a laser beam in the direction along thestriped opening 42 formed between the insulator masks 41.

According to this embodiment, the insulator masks 41 are formed on thesemiconductor layer 23 low in crystal defect density produced byselective growth using the insulator masks 4, which insulator masks 41are used for selective growth to form a waveguide structure for lasing.In this way, the optical loss in the waveguide structure can be reducedbelow that for the device of the first to tenth embodiments.Specifically, as compared with the crystal defect density in the rangeof 10⁴ to 10⁵/cm² of the nitride semiconductor layer (GaN layer) formedby homoepitaxial growth on the insulator masks 4 on the substrate, thecrystal defect density of the nitride semiconductor layers 5 to 8 formedby homoepitaxial growth on the insulator masks 41 based on the GaN layer23 formed by homoepitaxial growth can be reduced to the range of 10³ to10⁴/cm².

Consequently, by forming an optical crystal region by the nitridesemiconductor layers 5 to 7, it is possible to considerably reduce theinternal optical loss attributable to the light scattering loss due tothe crystal defect in the particular region (an optical active layer oran optical waveguide layer) and the gain loss due to the carriertrapping or the light absorption attributable to the deep level in theregion. Also, the width of the optical waveguide layer 5 can beregulated by adjusting the width of the window region (the size of theopening 42) between the insulator masks 41. In this way, it is possibleto construct either a gain-guided structure by setting the width of theoptical active layer 6 on the optical waveguide layer 5 to not less than5 μm or a refractive index-guided structure with a striped buriedheterostructure for guiding the wave with a real refractive-indexdifference by setting the width of the optical active layer 6 in therange of 1 to 3 μm. With the device having a refractive index-guidedstructure with a striped buried heterostructure according to thisembodiment, an operation is possible with lower threshold current thanin the gain-guided structure. The threshold current can thus be reducedto ⅓ to ¼ of the current required for the device of the gain-guidedstructure. This device performs the lasing operation with lowerthreshold current and higher efficiency than the conventional deviceformed by bulk growth, and the lasing wavelength of this device at roomtemperature is in the range of 410 to 430 nm.

A variation of this embodiment will be explained with reference to FIG.16C. The device can be fabricated in the same manner as shown in FIG.16A. After growing up to the layer 8, however, the layers 7 and 8 areetched into the shape of a ridge stripe using an insulator as shown inFIG. 16C. The bottom width of the ridge stripe is set to the range of 2to 9 μm. Further, a buried layer 12 is formed on each side of the ridgestrip using an insulator. The buried layer 12 can be either n-type GaNcurrent-blocking layer formed by selective growth or a layer buried witha dielectric insulator. The specification of the insulator masks 4, 41is identical to that for the device shown in FIG. 16A, and the buriedlayers 12 are formed as stripes extending substantially in parallel tothe striped opening 42. FIG. 16C shows a section taken in line L-L′ inFIG. 16B.

In the device shown in FIG. 16C, current can be injected effectivelyonly by the width of the ridge stripe transversely of the optical activelayer 6, thus realizing a refractive index-guided structure with acomplex refractive index difference. Also, the flow of the carriersinjected into the optical active layer 6 is limited by the gap betweenthe buried layers 12 as well as by the opening 42. Therefore, carrierscan be efficiently injected into the desired region of the opticalactive layer 6 for an improved emission efficiency. This device canperform the lasing operation with substantially the same low thresholdcurrent and high efficiency as the striped structure with a buriedheterostructure of the device shown in FIG. 16A, and the lasingwavelength of the device at room temperature is in the range of 410 to430 nm.

<Embodiment 12>

Still another embodiment of the invention will be described withreference to FIGS. 17A and 17B. The device is fabricated the same way asin the 11th embodiment. The difference, however, lies in that in thisembodiment, after forming up to the n-type GaN layer 23, two dummypatterns for selective growth are added outside of the insulator masks41 shown in FIG. 17A. The interval between three striped openings 42 isset in such a manner that the waveguide structure formed in the centralopening (i.e., central window region) of the insulator mask 41 is notcoalesced with the crystal layers formed on the dummy patterns. Also,the crystal layers formed on the dummy patterns are covered with aninsulator in order not to be supplied with current. The remaining stepsare similar to those for the 11th embodiment, and a longitudinal sectionof the device shown in FIG. 17A (the section taken in line M-M′ in FIG.17B) can thus be produced.

According to this embodiment, the crystallinity and the geometry areimproved by avoiding unusual crystal growth in the central the waveguidestructure thereby to form a more flat and smoother rectangular stripedstructure with a buried heterostructure. As a result, the laser beam canbe guided with lower loss, and the threshold current can be reduced toat least to the range of ⅔ to ½ of the threshold current of the 11thembodiment. This device performs the lasing operation with the lasingwavelength of 410 to 430 nm at room temperature.

A variation of this embodiment will be explained with reference to FIG.17C. This device is a compromise between the device of FIG. 16C and thatof FIG. 17A. Specifically, according to this embodiment, the p-type GaNoptical waveguide layer 7 and the p-type GaInN contact layer 8 areprocessed into a ridge stripe, on both sides of which buried layers 12are formed thereby to produce a semiconductor laser device having alongitudinal sectional structure as shown in FIG. 17C.

In the device shown in FIG. 17C, a waveguide structure can be formed ina ridge stripe structure located at the central portion of the p-typeGaN optical waveguide layer 7 having a crystallinity and a geometryimproved over those of the 11th embodiment. Further, this permits thelaser beam to be guided with a lower loss, and the threshold current canbe reduced at least to the range of ⅔ to ½ of the threshold current inthe device shown in FIG. 16C. This device performs the lasing operationwith the lasing wavelength in the range of 410 to 430 nm.

<Embodiment 13>

A further embodiment of the invention will be described with referenceto FIGS. 18A and 18B. The device is fabricated in the same manner asthat of the 11th embodiment shown in FIG. 16C, except that as shown inFIG. 18B, insulator masks 4 for selective growth are formed with atleast three striped openings formed as window regions 40 before formingthe layers 3 and 4. Insulator masks 41 are formed over the layers 4 insuch a manner as to cover the openings (window regions) 40 of theinsulator masks 4. The openings (window regions) 42 of the insulatormasks 41 are formed over the pattern of the insulator masks 4 (i.e., theregions sandwiched by the openings 40). The development of crystaldefects can be suppressed in two stages by using the insulators in twosteps of selective growth. Then, the crystal layers are coalesced on theinsulator masks 41 thereby to form an optical waveguide layer 5. Alongitudinal section (the section taken in line N-N′ in FIG. 18B) of thedevice can thus be produced as shown in FIG. 18A in exactly the same wayas in the 11th embodiment.

According to this embodiment, the crystal defect density in the opticalwaveguide layers 5, 7 and the optical active layer 6 can be reduced fora lower internal optical loss as compared with the devices of the 11thand 12th embodiments. In this way, the threshold current can be reducedto about one half that of the device shown in FIGS. 16C and 17C. Thisdevice accomplishes the lasing operation with the lasing wavelength inthe range of 410 to 430 nm at room temperature.

A variation of this embodiment will be explained with reference to FIG.18C. This device is a compromise between the device of FIG. 17C and thatof FIG. 18A. Specifically, dummy patterns are added to the stripedopenings 42 of the insulator masks 41.

With the device shown in FIG. 18C, a waveguide structure having a ridgestripe structure at the central portion of the p-type GaN opticalwaveguide layer 7 can be formed with a crystallinity and a geometryimproved over the device of FIG. 18A. Consequently, the laser beam canbe guided with a still lower loss, and the threshold current can bereduced to at least the range of ⅔ to ½ that for the device shown inFIG. 18A. This device performs the lasing operation with the lasingwavelength in the range of 410 to 430 nm at room temperature.

<Embodiment 14>

Still another embodiment of the invention will be described. This deviceis fabricated in a similar manner to at least one of those described inthe 11th to 13th embodiments, except that in this embodiment, thesubstrate 1 is made of n-type silicon carbide (α-SiC) with the substratesurface orientation in the (0001)C plane having a hexagonal symmetrystructure instead of a sapphire (α-Al₂O₃) substrate. A n-type GaN bufferlayer is formed on this substrate. Then, the device is fabricatedthrough the same steps as any one of the device structures in the 11thto 13th embodiments, thereby producing the device shown in FIGS. 16A to16C or FIGS. 18A to 18C.

According to this embodiment, the substrate has a conductivity, andtherefore can be mounted on a heat sink in such a manner that a n-sideelectrode is deposited by evaporation on the reverse side of thesubstrate and the surface of the substrate having a junction formedthereon by crystal growth constitutes the lower surface thereof. Currentcan thus be supplied from the p-side electrode on the upper surface ofthe substrate through nitride semiconductors to the n-side electrode onthe lower surface of the substrate. Further, the thermal conductivity ofthe SiC substrate is larger than that of the sapphire substrate, andtherefore the SiC substrate has a superior thermal radiation. The heatdissipation can thus be remarkably improved over the other embodiments.The present embodiment can produce a laser device which can operate at ahigher temperature than those of other embodiments. This device performsthe lasing operation with the lasing wavelength in the range of 410 to430 nm at room temperature.

<Conclusion>

The description that follows is a summarization of the knowledgeobtained on the semiconductor optical devices according to the presentinvention examined above with reference to the first to 14thembodiments.

In all the embodiments, the device is fabricated by a process comprisinga first step of forming a second region made of an insulating materialof amorphous structure on a first region having a crystal structure, asecond step of forming at least an opening in the second region forexposing the first region constituting a base, and a third step offorming a third region made of the crystal of a nitride semiconductormaterial on the second region in such a manner as to cover the secondregion, for example, in that order. This fundamental process offabricating the device is based on the architecture of the inventionthat a nucleus for crystal growth is formed only in at least an openingin the second region. Specifically, the nucleation region is limited tothe surface of the first region exposed on the bottom of the opening,and the nucleation density is increased at the particular position whileat the same time suppressing the nucleation on the second region. Forthe significance of the latter idea, refer to the discussion made inConclusion 3 relating to crystal growth.

According to an embodiment, a nitride semiconductor layer is used as afirst region formed by epitaxial growth on a sapphire substrate having acrystal structure of the hexagonal system, a silicon carbide substrateor on the (0001) plane of the crystal of any one of the above-mentionedsubstrates, and a second region is formed on the particular (0001)plane. Further, a nitride semiconductor crystal (GaN layer) is grown asa third region in such a manner as to cover the upper surface of thesecond region (i.e., the portion other than the opening)two-dimensionally. A nitride semiconductor crystal of a differentcomposition (AlGaInN) is formed by epitaxial growth on the third regionthereby to constitute an optical crystal region. A similar device can beproduced by forming the first region using a substrate made of GaAs,InP, InAs, GaSb, GaP, GaAsP or GaInAs having a zinc-blende structure,one of cubic systems, and by forming the second region on the particularsubstrate or on the (111) plane of the nitride semiconductor layerformed by epitaxial growth on the substrate. It should be noted herethat the optical crystal region of the conventional semiconductoroptical device, which is formed by using a sapphire substrate or siliconcarbide having a hexagonal crystal structure and by forming a nitridesemiconductor layer on the (0001) plane thereof by heteroepitaxialgrowth (or by bulk growth), has a crystal defect density in the range of10⁸ to 10¹¹/cm², while the crystal defect density in the optical crystalregion of the semiconductor optical device according to the presentinvention can be reduced to the range of 10⁴ to 10⁵/cm² or lessregardless of which crystal structure, the hexagonal system or the cubicsystem, is employed for the substrate. Further, in the method accordingto the 11th to 14th embodiments in which the first region constituted ofa nitride semiconductor layer is formed on a region made of aninsulating material of amorphous structure, the crystal defect densityof the third region formed on the upper surface of the second region(i.e., the portion other than the opening) or on the optical crystalregion formed on the third region can be reduced to the range of 10³ to10⁴/cm² or less.

With the semiconductor optical device according to the presentinvention, an optical crystal region (an optical waveguide layer, forexample) can be formed of a high-quality nitride semiconductor crystallow in defect density as described above, and therefore the optical loss(substantially proportional to the defect density) in the opticalcrystal region can be reduced. Specifically, in the case where a crystaldefect occurs in the optical crystal region, the resulting loss by lightscattering, light absorption due to a deep level and the carrier traplead to a gain loss in the particular optical crystal region. Accordingto the present invention, this gain loss in the optical crystal regionis suppressed. The internal optical loss in the optical crystal regioncan thus be considerably reduced. Therefore, the optical gain in theoptical crystal region is improved, thereby making possible a lasingoperation with low threshold current.

On the other hand, the third region of the semiconductor optical deviceaccording to the invention is grown in rectangle on the second regionand has a substantially flat surface of growth. As a result, it isgeometrically possible to smoothly realize, using a nitridesemiconductor, a gain-guided cavity with a gain distribution in thetransverse direction of an active layer (in the direction perpendicularto the cavity length) in a current-blocking layer, or a striped ridgestructure or a striped structure with a buried heterostructureconstituting a refractive index-guided cavity for guiding the wave infundamental transverse mode by setting a refractive index differencetransversely of the active layer.

The above-mentioned low crystal defect density and smooth rectangularstructure of the optical crystal region, which has been realized for thefirst time by the semiconductor optical device according to the presentinvention, improves the performance of a semiconductor laser device tosuch an extent that the threshold current for lasing is reduced to atleast one half that of the conventional devices and the internal quantumefficiency at least twice that of the conventional devices. The presentinventors have confirmed from various device configurations that thethreshold current can be reduced to as low as one-eighth that of theconventional devices. This improved performance is critical to thelow-threshold current, high-efficiency operation of a semiconductorlaser device.

Further, depending on the shape of the opening formed in the secondregion, it is possible to realize a phased array structure capable ofhigh-output operation in fundamental transverse mode or a verticalcavity structure for surface emitting suitable for reducing thethreshold current. In the former structure, an operation in longitudinalsingle mode can be achieved by forming a waveguide structure as a cavitymirror in each window region (opening) of the insulator mask. In thedevice having the configuration shown in the 11th to 14th embodiments,the threshold current for lasing can be reduced to ⅓ to ¼ that of theconventional devices having a gain-guided structure, by fabricating astriped refractive index-guided structure with a buried heterostructurehaving a real refractive index difference in the direction transverse ofan active layer or a ridge-striped refractive index-guided structurehaving a complex refractive index difference formed by etching,depending on the design width of the window region of the insulatormask.

In the device according to each of the embodiments, the lasingwavelength at room temperature is set to the wavelength of theblue-violet region in the range of 410 to 430 nm. The emission of greento violet not more than 600 nm in lasing wavelength is possible byappropriately setting the composition of the nitride semiconductor layermaking up the optical crystal region. For this method, refer to thereferences cited first above as the prior art.

INDUSTRIAL APPLICABILITY

In the semiconductor materials and the methods of fabrication thereofaccording to the invention, there is provided a body suitable forgrowing the crystal of a nitride semiconductor or a III-V compoundsemiconductor having a hexagonal crystal structure while at the sametime suppressing the defect density thereof to 10⁷/cm² or less.Therefore, the semiconductor device that already finds applicationsusing a III-V compound semiconductor having a crystal structure of thecubic system can be configured and used in practical applications withthe crystal of a nitride semiconductor or the crystal of a III-Vcompound semiconductor having a hexagonal crystal structure.

Further, with the semiconductor optical device constituting one of thesemiconductor devices according to the invention, the optical loss orthe carrier loss in the optical crystal region can be reduced. Forexample, the 100-hour continuous operation (CW oscillation) by asemiconductor laser device composed of a nitride semiconductor that hasso far been considered difficult is made possible by the invention. Asemiconductor laser device according to the invention, therefore, has anadditional effect of promoting the practical application of commercialequipments such as a high-definition DVD, a high-density MD or a laserdisplay using what is called a short-wavelength laser beam of blue-greento blue-violet.

What is claimed is:
 1. A semiconductor laser device comprising alight-emitting diode formed on a single-crystal substrate, wherein anoptical waveguide structure is formed on the surface of said substratein such a manner that multiple layered crystals are formed by epitaxialgrowth laterally from the ends of an insulator mask with the side ofeach of said ends as a base on a pattern having said insulator mask,said layered crystals are coalesced with each other on a central regionof said insulator mask thereby to configure an optical waveguide layercomposed of a single flat and smooth crystal layer of low defectdensity, and an optical active layer is formed on said optical waveguidelayer.
 2. A semiconductor laser device according to claim 1, wherein awaveguide structure is fabricated using a selective growth techniquecapable of limiting a nucleation region and improving nucleation densityby an insulator mask in such a manner that crystal layers having arectangular section grown in at least two window regions respectivelyare grown laterally from said window regions and are coalesced with eachother on said insulator mask thereby to form an optical waveguide, andsaid optical waveguide has a flat and smooth upper surface and has arectangular sectional structure with the sides thereof having a crystalplane perpendicular to the substrate surface, an optical active layerhaving a small band gap is formed on said rectangular optical waveguideformed by the coalescence of said crystal layers, and said opticalactive layer is configured of a double heterojunction structuresandwiched between optical waveguides having a large band gap.
 3. Asemiconductor laser device according to claim 1, wherein said opticalwaveguide structure includes a selected one of a gain-guided structurehaving a gain difference in the lateral direction of said optical activelayer by forming a current-blocking layer for limiting the currentpassage and a refractive index guided structure having a gain differenceand a refractive index difference in the lateral direction of saidactive layer.
 4. A semiconductor laser device according to claim 3,wherein in the case where said optical wave-guide structure is composedof a refractive index-guided structure, said refractive index-guidedstructure is configured of a waveguide structure having a refractiveindex difference capable of guiding the light stably only in afundamental lateral mode in each of vertical and lateral directions, andsaid device has a selected one of a buried stripe structure capable ofbeing formed with a real refractive index difference in the lateraldirection of the active layer and a ridge stripe structure capable ofbeing formed with a complex refractive index difference in the lateraldirection of said active layer.
 5. A semiconductor laser deviceaccording to claim 1, wherein said waveguide structure is configured ofa selected one of an optical waveguide and an optical active layer madeof an AlGaInN material of a nitride semiconductor.
 6. A semiconductorlaser device according to claim 1, wherein said optical active layer hasa selected one of a single quantum well structure and a multiple quantumwell structure configured of a quantum well layer.
 7. A semiconductorlaser device according to claim 6, wherein said optical active layer hasselected one of a single strained quantum well structure and a multiplestrained quantum structure configured of a strained quantum well layerhaving a lattice strain.
 8. A semiconductor laser device according toclaim 7, wherein said optical active layer is configured of a quantumbarrier and a strained quantum well having a lattice strain, and saidoptical active layer has a strain-compensation quantum well structurecompensating for the lattice strain amount over the entire area of saidoptical active layer.
 9. A semiconductor laser device according to claim1, wherein said single-crystal substrate has a hexagonal symmetrycrystal structure and is surrounded by the (0001)C plane or by the(11-20)A plane.
 10. A semiconductor laser device according to claim 9,wherein in the case where said optical waveguide structure on asubstrate has a surface orientation in the (0001)C plane, a waveguide isformed in the direction parallel or perpendicular to the (11-20)A planeof said substrate, and in the case where said optical waveguidestructure on a substrate has a surface orientation in the (11-20)Aplane, said waveguide is formed in the direction parallel orperpendicular to the (1-100)M plane of said substrate.
 11. Asemiconductor laser device according to claim 10, wherein saidsingle-crystal substrate is made of single-crystal sapphire (α-Al₂O₃) orsilicon carbide (α-SiC) having the (0001)C plane or the (11-20)A plane.12. A semiconductor laser device according to claim 1, wherein saidinsulator mask is made of an insulator of an amorphous structure.